CD fundamentals.


Subject: [2-1] How is the information physically stored?
(2004/02/20)

From _The Compact Disc Handbook, 2nd edition_ by Ken Pohlmann, 1992 (ISBN
0-89579-300-8):

"Write-once media is manufactured similarly to conventional playback-only
discs. As with regular CDs, they employ a polycarbonate substrate, a
reflective layer, and a protective top layer. Sandwiched between the
substrate and reflective layer, however, is a recording layer composed of
an organic dye. ....  Unlike regular CDs, a pre-grooved spiral track is
used to guide the recording laser along the spiral track; this greatly
simplifies recorder hardware design and ensures disc compatibility."

Your basic CD-R is layered like this, from top to bottom:

  [optional] label
  [optional] scratch-resistant and/or printable coating
  UV-cured lacquer
  Reflective layer (24K gold or a silver alloy)
  Organic polymer dye
  Polycarbonate substrate (the clear plastic part)

Yes, it's real gold in "green" and "gold" CDs, but if you hold a CD-R up to
a light source you'll notice that it's thin enough to see through (the gold
layer is between 50 and 100nm thick).  Something to bear in mind is that
the data is closest to the label side of the CD, not the clear plastic side
that the data is read from.  If the CD-R doesn't have a hard top coating
such as Kodak's "Infoguard", it's fairly easy to scratch the top surface
and render the CD-R unusable.


A pressed CD has raised and lowered areas, referred to as "lands" and
"pits", respectively.  A laser in the CD recorder creates marks in the
disc's dye layer that have the same reflective properties.  The pattern
of pits and lands on the disc encodes the information and allows it to be
retrieved on an audio or computer CD player.  See section (2-43) for
specifics.

Discs are written from the inside of the disc outward.  On a CD-R you can
verify this by looking at the disc after you've written to it.  The spiral
track on a 74-minute disc makes 22,188 revolutions around the CD, with
roughly 600 track revolutions per millimeter as you move outward from the
lead-in (23mm from the center) to the outer edge at 58mm.  If you "unwound"
the spiral, it would be about 5700 meters (3.5 miles) long.

The construction of a CD-RW is different:

  [optional] label
  [optional] scratch-resistant and/or printable coating
  UV-cured lacquer
  Reflective layer (aluminum)
  Upper dielectric layer
  Recording layer (phase change film, i.e. the part that changes form)
  Lower dielectric layer
  Polycarbonate substrate (the clear plastic part)

See the net references section for pointers to more data (especially
http://www.cd-info.com/).  You can find some nice drawings at
http://www.pctechguide.com/09cdr-rw.htm.  The various pages connected
to http://www.chipchapin.com/CDMedia/cdda5.php3 have some computations on
disc parameters.

The Philips document "Principles of Phase Change Recordings" at
http://www.licensing.philips.com/information/cd/rec/ has some nice drawings
and a very detailed explanation of how CD-RW works.


Subject: [2-2] What is XA? CDPLUS? CD-i? MODE1 vs MODE2? Red/yellow/blue book?
(2002/12/20)

A quick summary of standards and commonly used identifiers:

  Red Book    = physical format for audio CDs (a/k/a CD-DA)
  Yellow Book = physical format for data CDs
  Green Book  = physical format for CD-i
  Orange Book = physical format for recordable CDs
    Part I    = CD-MO (Magneto-Optical)
    Part II   = CD-WO (Write-Once; includes "hybrid" spec for PhotoCD)
    Part III  = CD-RW (ReWritable; originally called CD-E)
  White Book  = format for VideoCD (often written "VCD")
  Blue Book   = CD Extra (occasionally used to refer to LaserDisc format)
  CD Extra    = a two-session CD, 1st is CD-DA, 2nd is data (a/k/a CD Plus)
  MODE-1      = standard 2048-byte Yellow Book sectors, with error correction
  MODE-2      = 2336-byte sectors, usually used for CD-ROM/XA
  CD-ROM/XA   = eXtended Architecture; CD-ROM/XA MODE-2 defines two forms:
    FORM-1    = 2048 bytes of data, with error correction, for data
    FORM-2    = 2324 bytes of data, no ecc, for audio/video
  ISO-9660    = file layout standard (evolved from High Sierra format)
  Rock Ridge  = extensions allowing long filenames and UNIX-style symlinks
  CD-RFS      = Sony's incremental packet-writing filesystem
  CD-UDF      = industry-standard incremental packet-writing filesystem
  CD-Text     = Philips' std for encoding disc and track data on audio CDs

CD-ROM/XA is an extension to the Yellow Book Mode 2 standard.  It was
intended as a bridge between CD-ROM and CD-i (Green Book).

See http://www.licensing.philips.com/ if you want to buy copies of
the standards.  They're not cheap!  You can download some of them from
http://www.ecma-international.org/.  ECMA-119 describes ISO-9660, and
ECMA-130 sounds a lot like "yellow book" if you say it slowly.

For SVCD, see http://www.iki.fi/znark/video/svcd/overview/.  The discs
are a modified White Book format, using a 2x player and variable bit rate
MPEG-2 instead of MPEG-1 at 1x like VCD.

For HDCD, see http://www.hdcd.com/.  The discs are in Red Book format,
but the low bit of the audio has additional information encoded in it.
They sound good on a standard CD player, and better on an HDCD player.

SACD isn't really a CD format.  It can have a Red Book compliant layer
that is read by standard CD players, but to get the high-fidelity benefits
you need a special player.


Subject: [2-3] How do I know what format a disc is in?
(2001/07/09)

You can usually tell by looking at the packaging and/or the disc itself:

 - CD-DA discs will have a "Compact Disc Digital Audio" logo.
 - CD+G discs will have the words "CD Graphics" (and perhaps even
   CD-EG "Extended Graphics").
 - CD-i discs will have a "Compact Disc Interactive" logo.
 - VideoCD discs will have a "Compact Disc Digital Video" logo
   and/or the words "VideoCD".
 - PhotoCD discs will most likely say "Kodak PhotoCD" on them.
 - SVCD discs have a "Super Video CD" logo (the words "Super Video"
   under the standard CD logo).  The discs use one of the standard
   CD-ROM formats.
 - DVCD discs say "DVCD"??  [ can't find much info about DVCD ]
 - HDCD (High Definition Compatible Digital) have an "HDCD" logo.  See
   http://www.hdcd.com/.  The discs appear to use the standard Red Book
   format.
 - SACD (Super Audio Compact Disc) is relatively new.  The discs can
   have two layers, one of which is in Red Book audio format, the other
   in a DVD-like format offering higher fidelity.
 - DTS (Digital Theater Surround) CDs are just like normal CDs, but
   use DTS encoding instead of PCM.  See (2-34).

VideoCD is different from CD-Video (a/k/a "Compact Disc Video", or CD-V).
CD-V is an analog format, like LaserDisc, and the video can't be viewed
with a CD-ROM drive.

There are a few references to Compact Disc MIDI, or CD-MIDI.

See (4-46) for some comments on High Speed CD-RW.


Subject: [2-4] How does copy protection work?
(2002/04/01)

Copy protection (sometimes erroneously referred to as "copyright protection")
is a feature of a product that increases the difficulty of making an
exact duplicate.  The goal is not to make it impossible to copy -- generally
speaking, that can't be done -- but rather to discourage "casual copying"
of software and music.

The goal is *not* to conceal information from prying eyes; see section
(3-19) for information on encrypting data on a CD-ROM.

A separate but related issue is "counterfeit protection", where the publisher
wants to make it easy to detect mass-produced duplicates.  An example of
this is Microsoft's placement of holograms on the hubs of their CD-ROMs.
There are full CD pressing plants dedicated to creating counterfeit software
(the worst offender being mainland China), so this is a serious concern
for the larger software houses.

Copy protection on CD-ROMs used to be rare, but as the popularity of
CD recorders grew, so did the popularity of copy protection.  A large
percentage of games released in the past few years have been protected.

A more recent innovation is copy protection for audio CDs, inspired by
the rise of MP3 trading over the Internet.  This is more difficult to do,
because the protection must allow correct behavior on a CD player but
altered playback when being read by a CD-ROM drive.  The best that can be
accomplished is to force the user to play the music in an analog format
and then re-digitize it, resulting in an imperfect reproduction.

The article at http://news.cnet.com/news/0-1005-201-7320279-0.html is a
nice introduction to the issues.

Some people have questioned whether copy protection is legal.  In some
countries it may not be.  In the USA, the law allows "fair use" of
copyrighted material, but does not require that the content provider
make it easy for you to do so.  So while making a copy of a song for your
own private use may be legal, there is nothing in the law that requires
the publisher to make the material available in an unprotected format.
Copy protection has been around for many years -- some of the schemes
employed on the Apple II were remarkably elaborate -- and has never been
challenged on legal principle.

See http://overclockers.com/tips907/ for an article about why "fair use"
is a legal right rather than a constitutional right in the USA, and what
that means to you.  The article also has some interesting quotes from
the courts regarding the DMCA and DeCSS, notably this one: "We know of no
authority for the proposition that fair use, as protected by the Copyright
Act, much less the Constitution, guarantees copying by the optimum method
or in the identical format of the original."  In other words, arguing that
"fair use" means the publisher must allow you to make a perfect digital copy
(as opposed to a lower-quality digital or analog copy) is without merit.

The next sections discuss data and audio individually.


Subject: [2-4-1] ...on a data CD-ROM?
(2002/12/09)

There are several approaches.  An article with a good overview
of some popular protection technologies can be found at
http://www.tomshardware.com/storage/02q2/020617/index.html.
Another source is the "CD Protections" articles on
http://www.cdmediaworld.com/hardware/cdrom/cd_protections.shtml.

For anyone interested in protecting their own discs: don't bother.  Copy
protection, on the whole, does not work.  If you have a major application,
such as a game or CAD package, you may want to consider one of the
commercially licensed schemes listed later, or (heaven forbid) the use of a
dongle.  In general, though, if the disc can be read, then the contents
can be copied.  If you don't want somebody to make a copy of your stuff,
then you'd better encrypt it (3-19).

A simple and commonly seen technique is to increase the length of several
files on the CD so that they appear to be hundreds of megabytes long.
This is accomplished by setting the file length in the disc image to be
much larger than it really is.  The file actually overlaps with many
other files.  So long as the application knows the true file length,
the software will work fine.  If the user tries to copy the files onto
their hard drive, or do a file-by-file disc copy, the attempt will fail
because the CD will appear to hold a few GB of data.  (In practice this
doesn't foil pirates, because they always do image copies.  And, no, none
of the standard software provides a way to create such discs.)

One possible implementation, given sufficient control over the reader and
mastering software, is to write faulty data into the ECC portion of a data
sector.  Standard CD-ROM hardware will automatically correct the "errors",
writing a different set of data onto the target disc.  The reader then
loads the entire sector as raw data, without doing error correction.  If it
can't find the original uncorrected data, it knows that it's reading a
"corrected" duplicate.  This is really only viable on systems like game
consoles, where the drive mechanism and firmware are well defined.  This
can be defeated by doing "raw" reads.

A more sophisticated approach is to write special patterns of data to the
disc.  The stream of data that results, after EFM encoding, is difficult
for some recorders to reproduce successfully, apparently because they don't
choose correct values for the merging bits.  This is often referred to on
web sites as "writing regular EFM patterns" or "weak sectors".  See section
(2-43) for details on EFM.

A less sophisticated -- and no longer effective -- method is to press a silver
CD with data out beyond where a 74-minute CD can write.  Copying the disc
used to require hard-to-find CD-R blanks, but now it's easy to use an
overburned 80-minute disc (sections (3-8-1) and (3-8-3)).

The approach some PC software houses have taken is to use nonstandard
gaps between audio tracks and leave index marks in unexpected places.
These discs are uncopyable by most software, and it may be impossible
to duplicate them on drives that don't support disc-at-once recording
(see section (2-9)).  With the right reader and software, though, this
isn't much of a problem either.

A method that enjoyed some popularity was non-standard discs with a track
shorter than 4 seconds.  Most recording software, and in fact some recorders,
will either refuse to copy a disc with such a track, or will attempt to
do so and fail.  A protected application would check for the presence and
size of the track in question.  Some recorders may succeed, however, so
this isn't foolproof.  (In one case, a recorder could write tracks that
were slightly over three seconds, but refused to write tracks that were
only one second.  There may be a limit below which no recorder will write.)
In such cases, the pirates need to remove the explicit check from the
software itself.

Putting multiple data tracks interleaved with audio tracks on a CD will
confuse some disc copiers.  However, it's difficult to actually use the
data on those additional tracks.

Sometimes the copy of a disc will have a different volume label.  This
usually only happens with file-by-file copies, not disc image copies, so
checking the disc name is marginally useful but not very effective.

Modifying the TOC so that the disc appears to be larger than it really is
will convince some copy programs that the source disc is too large.

Some of the fancier technologies use non-standard pit geometry that cause
players to read the data differently on consecutive attempts.  Sometimes
the player sees a "1", sometimes a "0".  If, when reading the track, the
CD-ROM drive sees different data each time, the software knows that the
disc is an original.  A duplicate disc will return the same data reliably.
(So too will some CD-ROM drives... this technology is not without problems.)

Some programs will examine the disc to try to determine if it's a CD-R.
This doesn't work on all readers, and it's possible to disguise discs,
so this isn't very effective.

CloneCD (section (6-1-49)) can copy many copy protected discs without
trouble, given the right combination of reader and writer.  Its main
feature is "raw" reads and writes, which not all drives support.

The Laserlok system from http://www.diskxpress.com/ claims to be able to
prevent unauthorized disc duplication at a low cost.  It can be copied
by CloneCD.

An unrelated product called LaserLock, from MLS LaserLock International
(http://www.laserlock.com/) has similar features.  It can be copied by
CloneCD.

TTR Technology's DiscGuard (http://www.ttr.co.il/ or http://www.ttrtech.com/
claims to be able to write a signature onto pressed CDs and CD-Rs that is
detectable by all CD-ROM drives but isn't reproducible without special
hardware.  A program could use this for copy protection by checking for
the presence of the signature, and refusing to run if it's not there.

Sony DADC is promoting a similar product called Securom.  Some information
is at http://www.sonydadc.com/hotnews/secu_fra.htm.

Yet another variant is C-Dilla's SafeDisc.  They were bought by Macrovision
(http://www.macrovision.com/).  Their more recent product, SafeDisc 2,
was the first to feature "weak sectors".

Yet another variant is CD-Cops from Link Data Security
(http://www.linkdata.com/).


Subject: [2-4-2] ...on an audio CD?
(2002/10/21)

The challenge here is to create a disc that will play on a standard
audio CD player but be difficult to copy or "rip" into an MP3.  The
techniques making headlines in mid-2001 were developed by Macrovision
(2-4-3) and SunnComm (2-4-4).

The earliest form of audio CD copy protection was SCMS.  This only works
on recorders that support SCMS, specifically consumer-grade stand-alone
audio CD recorders.  "Professional" recorders, and recorders that attach
to computers, do not support SCMS.  See section (2-25).

Some CDs used a damaged TOC (Table of Contents -- see section (2-27))
that confused some CD-ROM drives and ripping software.  More recent schemes
attempt to modify the audio samples in ways that confuse CD-ROM drives into
playing static.  The next few sections describe these approaches in detail.

A web site at www.fatchucks.com used to have a list of suspected
copy-protected discs and some tips on what you can do to let the industry
know that copy protection isn't appreciated.  The web site appears to be
gone, but you can see an archived copy of it here:
http://web.archive.org/web/20031002104003/www.fatchucks.com/z3.cd.html

Many forms of copy protection violate the CD-DA standard, and so the discs
aren't allowed to use the official CD logo art.  However, many CDs don't
have the logo anywhere, so its absence doesn't prove anything.

A paper entitled "Evaluating New Copy-Prevention Techniques for Audio CDs"
by J.A. Halderman (available only in PostScript format) can be found at
http://crypto.stanford.edu/DRM2002/halderman_drm2002_pp.ps.  The paper was
submitted to the 2002 ACM Workshop on Digital Rights Management
(http://crypto.stanford.edu/DRM2002/prog.html).


Incidentally, if you're convinced that record companies and artists are
raking in huge piles of cash from every CD they sell, you might want to
take a look at an Electronic Musician article that talks about where the
money comes from and where it goes.  See:
http://industryclick.com/magazinearticle.asp?magazineid=33&releaseid=9554&magazinearticleid=132835&SiteID=15
(You may need to use IE; Netscape 4.7 for Linux couldn't view the site.)

Interesting figures: only about 16% of CDs sold make enough money for the
publishers to break even.  The ones that do make enough money have to pay
for the rest.  For the recording artists, only about 3% sell enough music
to get any royalties.  With figures like these, it's not surprising that
the industry is taking steps to combat piracy.

For more news & commentary, see:

 - http://www.latimes.com/technology/la-000093190nov22.story
 - http://www.modbee.com/24hour/entertainment/story/183508p-1775112c.html
 - http://www.siliconvalley.com/docs/news/svfront/cd121701.htm

For some messages about Sony's discs that can crash computers, see
http://slashdot.org/article.pl?sid=02/04/03/226233&mode=nested.  A later
article in MacUser noted that the Celine Dion disc _A New Day Has Come_
will lock up iMacs and require physically disassembling parts of the
machine to get the disc back out.  The article is
http://www.macuser.co.uk/macsurfer/php3/openframe.php3?page=/newnews/newsarticle.php3?id=1990


Subject: [2-4-3] ...on an audio CD (Macrovision - SafeAudio)
(2001/08/28)

In the first part of the year 2000, TTR Technologies announced a product
called MusicGuard (http://www.MusicGuard.com/) that claimed to prevent
duplication of audio CDs.  The product was withdrawn, but the technology
has resurfaced in mid-2001 as a product called SafeAudio from Macrovision
(http://www.macrovision.com/).

The basic idea is to create samples that sound like bursts of static, and
scramble the ECC data around to make it look like an uncorrectable error.
Audio CD players will interpolate the samples during playback, but CD-ROM
drives doing digital audio extraction generally won't.  The result is
a disc that plays back correctly on a CD player, but won't "rip" or copy
correctly on a CD-ROM drive.

Some relevant sites and news articles:

 - http://www.macrovision.com/solutions/newtech/audio/safeaudio.php3
 - http://www.newscientist.com/news/news.jsp?id=ns9999998
 - http://cnet.com/news/0-1005-200-6604222.html


This approach relies on an anachronism of CD-ROM drive construction.
There are two ways to play a CD on a computer, one analog, one digital.
The analog path sends the audio across a cable connected from the CD-ROM
drive to the sound card.  Most of the CD player software available on
computers works by telling the CD-ROM drive to start playing the CD
through the analog cable.  (This may not hold true for newer Macintoshes
-- it appears Mac OS 9 uses an entirely digital approach.  Some recent CD
player applications for the PC also do this.)

The digital path requires reading the "raw" audio samples off of the disc,
possibly modifying the data (e.g. changing the byte ordering) into something
appropriate for the sound card, and then writing them to the sound device.
Until a few years ago, most CD-ROM drives did this very poorly, in part
because the analog and digital data paths were logically distinct in the
designers' minds.  Audio CDs used the audio path, data CD-ROMs used the
digital path, and while you *could* send audio over the digital path there
didn't seem to be much value in doing so.  (See section (2-15) for some
additional notes.)

What Macrovision appears to be exploiting is the different handling of
uncorrectable errors in audio samples on the digital path vs the analog path.
When playing an audio CD in a CD player or CD-ROM drive, the analog path
is used.  This path deals with uncorrectable (E32) errors by examining the
samples that come before and after the error, and interpolating between them.
On a scratched-up CD, this means that, while you may not be hearing the
exact samples that were originally recorded, you won't notice any glitches
because they're smoothed over.  This feature is definitely not something
you'd want on a data CD-ROM -- interpolating pieces of your spreadsheet
is not going to help you.

In most CD-ROM drives, reading an audio sector with digital audio extraction
is handled the same way that reading a data sector is: uncorrectable
errors are left alone.  Instead of getting interpolated samples, you get
to hear the original, scratched-up audio.  This is why some CDs will play
back just fine on your computer, but will come out all scratched up when
you extract them with the same drive.  The errors are there either way,
but when using a desktop CD player the errors have been smoothed over by
the logic in the analog output path.

Some drives may use interpolation during DAE at lower speeds.  If so, it
should be possible to "rip" a track from a copy-protected disc by reducing
the extraction speed to 1x.

Some people have suggested that software could be used to perform the
interpolation on extracted music, stripping out the bits that the music
companies added in.  The trouble with this approach is that, once the data
has been extracted, the CIRC encoding is no longer visible.  It may not be
easy to tell where the glitches are.  For example, it should be possible
to create a low-level but rhythmic distortion that will be noticeable,
annoying, and difficult to identify automatically.

(It's possible that any software specializing in defeating the copy
protection would run afoul of the DMCA (Digital Millenium Copyright Act),
and the authors subject to fines and criminal prosecution.  Come to think
of it, the preceding discussion might be illegal.  For more information
about the DMCA, see http://www.eff.org/.)

How can you get a "clean" copy of a protected disc?  There are four basic
approaches, in order of least to most desirable:

(1) Record directly from the analog outputs of the drive, feeding the sound
into a sound card or outboard A/D converter.  Some fidelity will be lost
when converting from digital to analog and back again, which is what the
industry is counting on.

(2) It should be possible to play the disc on a CD player with an S/PDIF
connector, and get error-interpolated digital output.  If played into a
digital sound card or a CD recorder with an S/PDIF input, it should be
possible to capture an exact copy of the original.  Of course, it has
to be done at 1x, and the track breaks may have to be added manually,
making it a potentially tedious affair.  This might be avoidable on a CD-R
"dubbing deck", but inexpensive models will add SCMS to the set of things
to worry about.  Don't forget that generation loss is possible with CDs,
especially if you record from CD-Rs (due to their higher BLER rate),
so copying to CD-R and then extracting from CD-R requires some care.
See section (3-18).

(3) Some drives support an extension described in recent versions of the
ATA/ATAPI and SCSI MMC specifications.  This extension to the "READ CD"
command returns a set of flags indicating which bytes in an audio block
were not corrected at the C2 level (section (2-17).  An audio extraction
application with access to this information could do its own interpolation
across errors.  Some applications already make some use of this feature;
see http://www.feurio.com/English/faq/faq_vocable_c2error.shtml.  The "drive
check" feature of cdspeed (section (6-2-11)) reports on whether or not a
drive is capable of returning "C2 pointers".

(4) A CD-ROM drive with logic that interpolates uncorrectable errors during
DAE would allow copying and ripping with no additional effort required.


The success or failure of audio CD copy protection hinges upon two factors:
how effective is it at preventing "casual copying", and what sort of
problems do the legitimate owners of audio CDs encounter when playing
their discs?  Macrovision claims that their "golden ear" listeners were
not able to tell the difference, though the test might be biased if the
folks with the shiny lobes were using high-end CD players that did an
especially good job of concealing uncorrectable errors.

A legitimate technical concern is that the copy protection reduces the
effectiveness of the error correction.  Because some percentage of ECC is
now required for proper playback on a *clean* disc, the odds of scratches
and fingerprints causing audible degradation are increased.  In practice,
if the "static" samples are relatively few and far between, the difference
would be statistically insignificant.

One last piece of advice: do not assume that any disc that doesn't extract
cleanly is copy-protected.  There have been many, many postings on message
boards from people who think they have found a protected disc, or how
some specific piece of software can defeat the protection.  Start with
the more common reasons: the disc is dirty, the disc was poorly made, your
CD-ROM drive is not that great at audio extraction, you're using software
that isn't the best.  There are many reasons why ripping an audio track
might fail.  People have been having trouble getting clean audio for years.
See section (3-3) for some advice if you're having trouble.

Certain web sites (notably cdfreaks.com) have been reporting that a
replacement CDFS.VXD will fix everything.  However, doing the audio
extraction in a VXD instead of an EXE makes no difference.  So far, none
of the sites that have claimed victory list a single SafeAudio-protected
disc that was copied, most likely because -- at the time this was written
-- there weren't any discs known to use SafeAudio.  (This phenomenon is
not unheard-of; Sega's Dreamcast discs were widely reported to be copyable
by a means that was quickly determined to be utterly ridiculous.)  If the
widely-touted CDFS.VXD is in fact a hijacked Plextor driver, then it may
well use technique #3 mentioned above, but would only work on a drive that
supported the extended READ CD feature.


Subject: [2-4-4] ...on an audio CD (SunnComm - MediaCloQ and MediaMax CD3)
(2005/12/09)

SunnComm (http://www.sunncomm.com/) has a product called "MediaCloQ".
It was used to protect the album _A Tribute to Jim Reeves_ by Charley Pride
in mid-2001.  The results were inconclusive: clean versions of the tracks
appeared on the net, but SunnComm claimed they came from an unprotected disc
released on Australia.  Their plan was to alleviate "fair use" concerns by
allowing users to download MP3 versions of the songs after they registered
the original.  Some articles:

 - http://news.cnet.com/news/0-1005-200-5924584.html
 - http://www.zdnet.com/zdnn/stories/news/0,4586,5082954,00.html
 - http://news.cnet.com/news/0-1014-201-7311791-0.html

Some early stories indicated that BMG Entertainment was considering the use
of this product.  Sony-BMG did eventually use SunnComm products on several
CDs.  See http://www.zdnet.com/zdnn/stories/news/0,4586,5094925,00.html.

The idea behind this protection is to make it hard for CD-ROM drives to
identify the disc as being an audio CD.  The disc is multisession, and
uses a hacked TOC, so track rippers and disc copiers have trouble dealing
with it.  SunnComm hasn't publicly stated any details.

In August 2001, SunnComm announced v2.0 of their product, but didn't
provide specific details.

In mid-2003, SunnComm announced "MediaMax CD3", a fancier implementation that
allows computer users to play the CD through software supplied on the disc.
The software installs a memory-resident driver that prevents CD ripping from
working on protected CDs.  The protection can be foiled on Windows PCs by
simply holding down the shift key for several seconds while inserting the CD.
See http://www.cs.princeton.edu/~jhalderm/cd3/ for a detailed analysis.
SunnComm announced they were going to sue the Princeton researcher, but
quickly backed off.

In December 2005, following the XCP disaster (see section (2-4-10)), a
flaw was discovered in MediaMax v5 that could allow malicious software to
gain control of an affected computer.  http://sonybmg.com/mediamax/ has a
"consumer advisory" regarding the problem, including a list of affected
CDs and links to a patch and uninstaller on the sunncomm.com web site.
It was subsequently determined that the patch was flawed; see
http://news.bbc.co.uk/1/hi/technology/4511042.stm.


Some personal notes on SunnComm's protection of the Charley Pride disc,
including the steps I took to get a clean copy:

The packaging is labeled with the SunnComm logo, and states, "This audio
CD is protected by SunnComm(tm) MediaCloQ(tm) Ver 1.0.  It is designed
to play in standard audio CD players only and is not intended for use in
DVD players."  However, my DVD player was able to play the disc after
overcoming some initial confusion.

The disc itself has an unusual construction.  There is a heavy band at about
the point where the music stops, and thin bands between tracks.  These appear
to be purely decorative (and, I'm told, increasingly common on non-protected
discs).  Some images are available on http://www.fadden.com/cdrpics/.

A computer running Win98SE with a Plextor 40max CD-ROM drive saw the
disc as having two sessions and 16 data tracks.  My CD player only saw 15
audio tracks.  This feature alone makes the disc difficult to rip or copy,
because the software doesn't see any audio tracks, and a CD-R copy would be
full of tracks that even a CD player would see as data.  Another machine,
with a Plextor 12/20 and a slightly different set of software, seemed
to have a lot of trouble figuring out what the disc was.  It eventually
sorted things out, but I get the sense the disc has been tweaked in ways
that confuse the drive firmware.

I tried using "Session Selector" to select the first session and then
access the tracks.  This resulted in a Plextor 8/20 CD recorder becoming
unusable until a reboot.  I'd guess the firmware got confused.

The next thing I tried was to crank up CDRWIN v3.7a (section (6-1-7)),
and extract some tracks using my Plextor 12/20.  No dice -- the display
showed 15 unselectable tracks and 1 MODE-2 data track.

Next, I tried the "Extract Disc/Tracks/Sectors" function, selected "Extract
Sectors", chose "Audio-CDDA (2352)" for the data type, and entered a
nice range (0 to 300000, where each audio sector is 1/75th of a second).
This choked when trying to read starting at block 173394, so I tried again
stopping at 173390.  This resulted in a rather large WAV file, which
I opened with Cool Edit -- revealing the entire contents of the disc,
plain and clear.  Playback revealed no audible defects.

I believe this worked because the sector extraction function ignores
track and session boundaries, and just pulls the blocks straight off.
Losing the track markers is annoying, but it's easy to add them back with
something like CDWave (section (6-2-16)).

(FWIW, this same approach did *not* work for the _My Private War_ disc
with the damaged TOC, described in (2-4-2).  It would probably not be
of help with a SafeAudio disc either.)

"zEEwEE" came up with a complicated but enlightening scheme for
side-stepping the protection on discs with damaged second TOCs.  It has the
advantage of allowing you to use standard tools, such as Exact Audio Copy
(section (6-2-12)), which keeps the track breaks and can do fancy tricks
to get the best extraction quality.  The method involves making the outer
rim of the disc unreadable to the CD-ROM drive by drawing on it with a
dry-erase marker or adding an adhesive sticker.  This method, first posted
in August of 2001, resulted in a flurry of media attention in May of 2002.


Subject: [2-4-5] ...on an audio CD (Midbar Tech - Cactus Data Shield)
(2002/02/13)

Midbar Tech Ltd (http://www.midbartech.com/) appears to have two different
schemes under the "Cactus Data Shield" brand.  (The web site shows three
now: CDS100, CDS200, and CDS300.)  The first uses a non-standard TOC.
The position of the lead-out and the length of the last track were
tweaked, resulting in a disc that appears to be only 28 seconds long.
The alterations didn't confuse all CD-ROM drives, and it has been reported
that some Philips CD players couldn't play the discs.  BMG Entertainment
reportedly tried it and abandoned it.

In late 2001, Midbar Tech announced a different approach.  A US patent
(http://www.delphion.com/details?&pn=US06208598__) describes the invention.

The approach appears to involve inserting frames of bogus control information
into a relatively constant part of the CD audio stream.  During playback,
the extra frames are skipped.  A disc copy or digital stream on an S/PDIF
output will include the bogus frames, and when written to CD-R the extra
control information won't be included.  The result is bad samples that only
appear in copies.

News articles:

 - http://news.com.com/2100-1023-835841.html
 - http://www.newscientist.com/news/news.jsp?id=ns99991105
 - http://www.cnn.com/2001/TECH/ptech/08/08/cd.copy.protection.reut/index.html

The difficulty in copying such a disc depends on how the stream of audio
samples appears.  In news articles the company claims that the scheme
can defeat method #2 described in section (2-4-3), in which the S/PDIF
connector of a CD player is used to get an error-interpolated digital
stream.  That suggests that the bogus data doesn't appear as uncorrected
data, but rather as valid data that is suppressed on the analog outputs.
This would seem to make digital copying difficult, but it would also make
any form of digital playback impossible.

No specific disc titles have been announced, but Sony has reportedly
released a few titles in eastern Europe that use this.


Some personal notes on the early version (CDS100?) of the Cactus Data
Shield: I bought a copy of _My Private War_, by Phillip Boa & The Voodoo
Club, from an online retailer.  The disc is labeled "Kopiergeschützte CD -
nicht am pc abspielbar" which translates literally to "copy-protected CD
- not at the PC playable".  Supposedly this is one of the BMG discs that
was protected with Midbar's first product.

The Plextor Plextools utility saw it as a single-session audio CD with
13 tracks, but when I asked it to play the disc it only saw the first
28 seconds of the first track, and stopped after playing just that much.
My Panasonic CD "boom box" also thought the disc was only 28 seconds long,
but it happily played past that point, and would let me select any track.

The page at http://uk.eurorights.org/issues/cd/docs/natimb.shtml has an
analysis of the CD _White Lilies Island_ by Natalie Imbruglia.

http://www.cdrinfo.com/Sections/Articles/Specific.asp?ArticleHeadline=Cactus%20Data%20Shield%20200&index=0
has a very thorough examination of a CDS200 disc.  Recommended reading.


Subject: [2-4-6] ...on an audio CD (Key2Audio / Sony DADC)
(2001/09/26)

This was used to protect promotional copies of the Michael Jackson
single "You Rock My World".  See http://www.key2audio.com/ for product
information.

News articles:

 - http://news.cnet.com/news/0-1005-200-7284684.html

The technology is designed to make the discs unrecognizeable to CD-ROM
drives.  According to the web pages, the product is licensed through
Sony DADC.


Subject: [2-4-7] ...on an audio CD (BayView Systems - Duolizer)
(2001/09/26)

The "Duolizer" system splits music into two pieces.  The bulk of the
music is on the CD, but a small but essential piece is streamed from a
secure server over the Internet.  The idea is to allow music publishers to
distribute songs to the media and retail outlets ahead of scheduled releases.
This was a response to songs appearing in MP3 form on the Internet before
the CDs went into distribution.

See http://www.bayviewsystems.com/solutions/duolizer.htm for product info.

News articles:

 - http://news.cnet.com/news/0-1005-200-7132601.html

This scheme can't be used for general CD protection, because if the music can
be played on a computer at all, it can be captured with a program like Total
Recorder (http://www.HighCriteria.com/).  It will be reasonably effective
for promotional copies of songs, though, where the goal is to prevent people
from walking away with copies of the discs.

As an added bonus, because the music is streamed from a central location,
it could have a digital watermark added.  If (say) somebody at a radio
station made an MP3 copy, it might be possible to trace the source of the
MP3 file back to the source.  There is nothing on the product pages to
suggest that such a scheme is currently in place.


Subject: [2-4-8] ...on an audio CD (Sanyo)
(2001/09/29)

Sanyo has joined the growing list of companies to announce CD copy
protection.  It's not clear if this is their own scheme or one licensed
from another company.

News articles:

 - http://www.cnn.com/2001/TECH/ptech/09/24/sanyo.cd.rom.protection.idg/index.html


Subject: [2-4-9] How does the Doc-Witness OpSecure CD-ROM work?
(2002/08/22)

The disc has an embedded secure micro (like a smart card) that is activated
when the laser light strikes a photodetector.  The light is converted to
electrical impulses, the impulses drive the chip, and if all goes well
the results are presented to the drive via an embedded light-emitting diode.

Making an exact duplicate of the disc would be very difficult.  It's unclear
whether this technology actually makes it harder to get a working copy
of the contents.  The scheme seems to essentially be a combination of an
"uncopyable" disc and a hardware dongle, both of which have been around
for years (neither of which have brought an end to piracy).

The company's web site is http://www.doc-witness.com/.

News articles:

 - http://www.technologyreview.com/articles/prototype10902.asp?p=7


Subject: [2-4-10] What's the Sony BMG rootkit (First 4 Internet XCP)?
(2005/11/29)

A "rootkit" is a bit of software that changes the way your system works,
usually for malicious purposes.  Sony BMG included one with some audio
CDs released in late 2005.

The software in question is "XCP Content Management" from First 4 Internet
Ltd (http://www.first4internet.com/).  It uses a combined audio CD and
CD-ROM format.  When placed in a CD-ROM drive on a Windows system, it
uses the autorun feature to install itself.  XCP includes anti-piracy
technology that acts to prevent you from copying it, and cloaking
technology to prevent you from seeing it.  If you manage to find it, and
try to remove it, it disables your CD-ROM drive.

(As with other technologies of this type, disabling autorun or holding
down the shift key while loading a CD will prevent the copy protection
from loading.  Because this protection is difficult to remove you must
be very careful when handling Sony music CDs on your computer.)

This produced a tremendous backlash against Sony BMG.  Besides the usual
objections to this sort of thing -- installing software that prevents your
system from functioning normally -- the rootkit could be used by other bits
of adware/spyware to conceal themselves.  (It was used by enterprising
game cheats to circumvent World of Warcraft's elaborate anti-cheating
system, and a couple of viruses were using it to conceal themselves.)

After news of XCP became widely known, Sony BMG began offering a software
download on its site that would identify affected systems by removing
the cloaking, but wouldn't remove the rootkit entirely.  You could get
the patch by filling out a marketing survey that -- according to Sony's
privacy policy -- could lead to having your e-mail address added to their
mailing lists.

Sony BMG eventually made an uninstaller available, but only if you
made some educated guesses on their web site and jumped through some
ridiculous hoops:
http://www.sysinternals.com/blog/2005/11/sony-you-dont-reeeeaaaally-want-to_09.html

It turned out the web-based uninstaller created security vulnerabilities,
causing yet more problems.  Some notes here:
http://blogs.washingtonpost.com/securityfix/2005/11/sony_uninstall_.html

There is some network activity associated with the rootkit.  It appears to
be connecting to a Sony web site to look for updated content.  There is
some speculation that this could be used for tracking purposes, though
Sony denies that they are doing so.

A class-action lawsuit was filed on behalf of residents of the state
of California (USA) in November 2005, and similar actions were planned
elsewhere.

Use of the technology was suspended in November 2005 in response to
public pressure.  Later that month, after the various security problems
became prominent, Sony BMG elected to recall all XCP-protected CDs.

News articles:

 - http://sfgate.com/cgi-bin/article.cgi?file=/news/archive/2005/11/02/financial/f160614S41.DTL
 - http://sfgate.com/cgi-bin/article.cgi?file=/c/a/2005/11/11/MNGFMFMNV61.DTL&type=tech
 - http://www.eff.org/deeplinks/archives/004117.php
 - http://blogs.washingtonpost.com/securityfix/2005/11/the_bush_admini.html
 - http://today.reuters.co.uk/news/newsArticle.aspx?type=technologyNews&storyID=2005-11-11T183106Z_01_MOL166114_RTRIDST_0_TECH-SONY-COPYPROTECTION-DC.XML&archived=False

Nice summary of the whole debacle:

 - http://www.businessweek.com/technology/content/nov2005/tc20051129_938966.htm

List of affected CDs:

 - http://cp.sonybmg.com/xcp/english/titles.html
 - http://www.eff.org/deeplinks/archives/004144.php

Technical info:

 - http://www.sysinternals.com/blog/2005/11/more-on-sony-dangerous-decloaking.html
 - http://www.f-secure.com/v-descs/xcp_drm.shtml


Subject: [2-5] What's a multisession disc?
(2005/02/07)

A session is a recorded segment that may contain one or more tracks of any
type.  The CD recorder doesn't have to write the entire session at once --
you can write a single track, and come back later and write another -- but
the session must be "closed" before a standard audio CD or CD-ROM player
will be able to use it.  Additional sessions can be added until the *disc*
is closed or there's no space left.

This provides a simple and fairly reliable way to write some data to
a disc now and still be able to add more later.  The trouble with using
multiple sessions is that, every time you write a chunk of data, you incur
a fairly substantial amount of overhead: 23MB after the first session,
and 14MB for every subsequent session.  This overhead lead to the
development of "packet writing", which allows drag-and-drop recording,
but works in an entirely different way (see section (6-3)).

Multisession writing was first used on PhotoCD discs, to allow additional
pictures to be appended to existing discs.  Today it's most often used
with "linked" multisession discs, and occasionally for CD-Extra discs.
These require a bit more explanation.

When you put a data CD into your CD-ROM drive, the OS finds the last
closed session on the disc and reads the directory from it.  (Well,
that's how it's supposed to work.  On some older operating systems and
CD-ROM drives, you may get different results.)  If the CD was written in
ISO-9660 format -- most store-bought CD-ROMs are -- the directory entries
can point at any file on the CD, no matter which session it was written in.

Most of the popular CD creation programs allow you to "link" one or more
earlier sessions to the session currently being burned.  This allows the
files from the previous sessions to appear in the last session without
taking up any additional space on the CD (except for the directory entry).
You can also "remove" or "replace" files, by putting a newer version into
the last session, and by not including a link to the older version.

In contrast, when you put an audio CD into a typical CD player, it only
looks at the first session.  For this reason, multisession writes don't
work for audio CDs, but as it happens this limitation can be turned into
an advantage.  See section (3-14) for details.  This limitation does *not*
mean you have to write an entire audio CD all at once; see section (2-9)
for an overview of track-at-once writing.

(Some audio CD players do seem to be able to recognize all of the tracks on
a multisession audio disc.  Most do not.  The only way to know for sure is
to try and see.  If you are planning to give an audio CD you create to
others, it would be wise to write it in a single session.)

Note that mixing MODE-1 (CD-ROM) and MODE-2 (CD-ROM/XA) sessions on a
single disc isn't allowed.  You could create such a thing, but many CD-ROM
drives will have a hard time recognizing it.

See also http://www.roxio.com/en/support/cdr/multisession.html, which goes
into more depth.

On a Macintosh, discs written in HFS or HFS+ format cannot link files back
to earlier sessions.  Adding a new session will cause the previous session
to disappear.

Quick recap: if you want to write some data to a CD-ROM now, and some
more later, you write a single data track in multiple sessions (or with
packet writing).  If you want to write some audio tracks to a CD now,
and some more later, you write multiple audio tracks in a single session.


Subject: [2-6] What are subcode channels?
(2002/12/10)

There are eight subcode channels (P,Q,R,S,T,U,V,W).  The exact method of
encoding is discussed in section (2-43), but it's really only important
to note the data is distributed uniformly across the entire CD, and each
channel can hold a total of about 4MB.

The P subcode channel identifies the start of a track, but is usually
ignored in favor of the Q channel.

The Q subcode channel includes useful information, which can be read and
written on many recorders.  The user data area contains three types of
subcode-Q data: position information, media catalog number (MCN), and
ISRC code.  Other forms are found in the lead-in, and are used to enable
multisession and describe the disc TOC (table of contents).

The position information is used by audio CD players to display the current
time, and has track/index information.  This can be controlled when doing
Disc-At-Once recording.

The ISRC (International Standard Recording Code) is used by the recording
industry.  It states the country of origin, owner, year of issue, and
serial number of tracks, and may be different for each track.  It's
optional; many CDs don't use this.  The media catalog number is similar,
but is constant per disc.  Note these are different from the UPC codes.

The R-W subcode channels are used for text and graphics in certain
applications, such as CD+G (CD w/graphics, supported by SegaCD among
others).  A new use has been devised by Philips, called ITTS.  It enables
properly equipped players to display text and graphics on Red Book audio
discs.  The most recent result of this technology is "CD-Text", which
provides a way to embed disc and track data on a standard audio CD.


Subject: [2-7] Are the CD Identifier fields widely used?
(2002/12/20)

Not many publishers use them, and not all devices can read all of the fields.

Programs that identify audio CDs automatically don't rely on an embedded
serial number.  Instead, they compute an ID based on the quantity and
positions of the audio tracks, measured down to 1/75th of a second.
http://www.gracenote.com/ has a collection of CD information.


Subject: [2-8] How long does it take to burn a CD-R?
(2001/05/31)

It depends on how much data you're going to burn, and how fast your drive is.
Burning 650MB of data takes about 74 minutes at 1x, 37 minutes at 2x, and
19 minutes at 4x, but you have to add a minute or two for "finalizing"
the disc.  Remember that single speed for CD-ROMs is 150KB/sec, double
speed is 300KB/sec, and so on.

If you have half the data, it will finish in (about) half the time.  If you
record the same thing twice as fast, it will finish in (about) half the time.

Most CD recording speeds are linear, i.e. recording at 12x is twice as fast
as recording at 6x.  If the drive uses a PCAV mechanism (see section (5-22))
the speed varies depending on which part of the disc you're recording.
If a "20x" drive uses PCAV to get 12x at the start of the disc and 20x
near the outside, you know that burning 60 minutes of audio will take
somewhere between about 5 minutes and about 3 minutes.


Subject: [2-9] What's the difference between disc-at-once and track-at-once?
(2002/06/24)

There are two basic ways of writing to a CD-R.  Disc-at-once (DAO) writes
the entire CD in one pass, possibly writing multiple tracks.  The entire
burn must complete without interruption, and no further information may be
added.

Track-at-once (TAO) allows the writes to be done in multiple passes.  There
is a minimum track length of 300 blocks (600K for typical data CDs), and a
maximum of 99 tracks per disc, as well as a slight additional overhead
associated with stopping and restarting the laser.

Because the laser is turned off and on for every track, the recorder leaves
a couple of blocks between tracks, called run-out and run-in blocks.
If done correctly, the blocks will be silent and usually unnoticeable.
CDs with tracks that run together will have a barely noticeable "hiccup".
Some combinations of software and hardware may leave junk in the gap,
resulting in a slight but annoying click between tracks.  Some drives
and/or software packages may not let you control the size of the gap
between audio tracks when recording in track-at-once mode, leaving you
with 2-second gaps even if the original didn't have them.

Many recorders, starting with the venerable Philips CDD2000, allow
"session-at-once" (SAO) recording.  This gives you disc-at-once control
over the gaps between tracks, but allows you to leave the disc open.
This can be handy when writing CD Extra discs (see section (3-14)).

There are some cases where disc-at-once recording is required.  For
example, it may be difficult or impossible to make identical backup copies
of some kinds of discs without using disc-at-once mode (e.g. copy-protected
PC games).  Also, some CD mastering plants may not accept discs recorded in
track-at-once mode, because the gaps between tracks will show up as
uncorrectable errors.

The bottom line is that disc-at-once recording gives you more control over
disc creation, especially for audio CDs, but isn't always appropriate
or necessary.  It's a good idea to get a recorder that supports both
disc-at-once and track-at-once recording.


Subject: [2-10] Differences between recording from an image and on-the-fly?
(1998/12/20)

Many CD-R creation packages will give you a choice between creating a
complete image of the CD on disk and doing what's called "on-the-fly"
writing.  Each method has its advantages.

Disc image files are sometimes called virtual CDs or VCDs (not to be
confused with VideoCD).  These are complete copies of the data as it will
appear on the CD, and so require that you have enough hard drive space to
hold the complete CD.  This could be as much as 650MB for CD-ROM or 747MB
for an audio disc when using 74-minute blanks.  If you have both audio and
data tracks on your CD, there would be an ISO-9660 filesystem image for the
data track and one or more 16-bit 44.1KHz stereo sound images for the audio
tracks.

(On the Mac, you might instead use an HFS filesystem for the data track.
You can create the image with Mac CD recording software, or create it as a
DiskCopy image file and then burn the data fork under a different OS.)

On-the-fly recording often uses a "virtual image", in which the complete
set of files is examined and laid out, but only the file characteristics
are stored, not the data.  The contents of the files are read while the CD
is being written.  This method requires less available hard drive space and
may save time, but increases the risk of buffer underruns (see (4-1)).
With most software this also gives greater flexibility, since it's easier
to add, remove, and shuffle files in a virtual image than a physical one.

A CD created from an image file would be identical to one created with
on-the-fly recording, assuming that both would put the same files in the
same places.  The choice of which to use depends on user preference and
hardware capability.


Subject: [2-11] How does an audio CD player know to skip data tracks?
(1999/04/11)

There are subcode flags in the Q channel for each track:

  Data
    If set, the track contains data; if not, the track contains audio.
  Digital Copy Permitted
    Used by SCMS.  Set to allow copies, clear to prevent them.
  Four-Channel Audio
    The Red Book standard allows four-channel audio, though very few
    discs have ever been made that use it.
  Pre-Emphasis
    Set if the audio was recorded with pre-emphasis.

The last two are rarely used.


Subject: [2-12] How does CD-RW compare to CD-R?
(2001/07/06)

CD-RW is short for CD-Rewritable.  It used to be called CD-Erasable (CD-E),
but some marketing folks changed it so it wouldn't sound like your
important data gets erased on a whim.  The difference between CD-RW and
CD-R is that CD-RW discs can be erased and rewritten, while CD-R discs are
write-once.  Other than that, they are used just like CD-R discs.

Let me emphasize that: they are used just like CD-R discs.  You can use
packet writing on both CD-R and CD-RW, and you can use disc-at-once audio
recording on both CD-R and CD-RW.  Some software may handle CD-RW in a
slightly different way, because you can do things like erase individual
files, but the recorder technology is nearly identical.

CD-RW drives use phase-change technology.  Instead of creating "bubbles"
and deformations in the recording dye layer, the state of material in the
recording layer changes from crystalline to amorphous form.  The different
states have different refractive indices, and so can be optically
distinguished.

These discs are not writable by standard CD-R drives, nor readable by most
older CD readers (the reflectivity of CD-RW is far below CD and CD-R, so an
Automatic Gain Control circuit is needed to compensate).  Most new CD-ROM
drives do support CD-RW media, but not all them will read CD-RW discs at
full speed.

A few older audio CD players and many new ones can handle CD-RW discs, but
many can't.  If you want to create audio CDs on CD-RW media, make sure that
your player can handle them.

All CD-RW recorders can write to CD-R media, so the only reason not to buy
a CD-RW recorder is price.  Some Internet sites like to put the devices in
completely separate categories, calling them "CD recorders" and "CD
ReWriters", but the differences between them don't really merit such a
distinction.  Think of a "CD ReWriter" as a CD recorder that can also make
use of CD-RW media.

Oddly enough, it may be easier for a DVD drive to read CD-RW discs than
CD-R discs, because of the way the media is constructed.

CD-RW media is more expensive than CD-R, but recent price reductions have
narrowed the gap considerably.  There is a limit to the number of times an
area of the disc can be rewritten, but that number is relatively high (the
Orange Book requires 1000, but some manufacturers have claimed as much as
100,000).  Of course, this is under laboratory conditions.  If you don't
handle the disc carefully, you will add scratches, dirt, fingerprints,
and other obstacles that make the disc harder for the drive to read.

It appears that CD-RW discs have speed ratings encoded on them, so discs
that are only certified for 2x recording can't be written to at 4x (or,
for that matter, 1x).  To make things more complicated, different media
is required for high-speed CD-RW drives (those that exceed 4x).

If you're trying to decide if you want a drive that supports CD-RW, see
section (5-16).


Subject: [2-13] Can DVD players read CD-Rs?
(2001/11/28)

The only discs that a DVD player is guaranteed to read are DVD discs.
Support for CD-ROM, CD-R, and CD-RW may be included, but is by no means
guaranteed.

CD-R was designed to be read by an infrared 780nm laser.  DVD uses a
visible red 635nm or 650nm laser, which aren't reflected sufficiently
by the organic dye polymers used in CD-R media.  As a result, many DVD
players can't read CD-R media.  Some DVD players come with two lasers so
that they can read CD-R.  For a technical discussion, see
http://www2.osta.org/osta/html/cddvd/intro.html and
http://www.emedialive.com/EM1998/bennett3.html
(web archive: http://web.archive.org/web/20040224114428/http://www.emediapro.com/EM1998/bennett3.html).

CD-RW discs have a different formulation, and may work even on players that
can't handle CD-R media.  If CD-R media doesn't work, try copying the
disc to CD-RW instead (assuming your recorder supports CD-RW).

Some DVD-ROM drives may be unable to read multisession discs.  In general,
though, DVD-ROM drives (as opposed to DVD players) are able to read
CD-R media.

If the box doesn't say that something is supported, assume that the
feature isn't.  Look for the MultiRead or MultiPlay logos, which indicate
that the DVD player or DVD-ROM drive can read CD-R and CD-RW.

See also "Is XXX compatible with DVD" in the DVD FAQ:
  http://www.dvddemystified.com/dvdfaq.html#2.4.3
  http://www.dvddemystified.com/dvdfaq.html#2.4.4


Subject: [2-14] Should I buy a DVD recorder instead?
(2007/07/08)

Your best bet is to get a "combo" drive that records on CDs as well.
With recent cost reductions to DVD hardware, there's no real reason to buy
a drive that only handles CDs or only handles DVDs (and in fact they're
increasingly difficult to find).

CDs are quickly surpassing the venerable 3.5" floppy disk as the most
universal physical media.  DVD-ROM drives and DVD players weren't as
successful initially as some in the industry had hoped -- near the end
of 2000, one of the major computer sellers was offering an "upgrade"
on their systems from DVD-ROM drives to CD recorders.  These days it's
hard to buy a computer that doesn't support all formats.

DVD-R recorders and media were initially very expensive, but eventually
came down to consumer levels.  An example: electroweb.com was, as of
early February '98, selling a Pioneer CDVR-S101 DVD-Recordable Drive for
US$18000.  In June '99, the same site had a Pioneer CDVR-S201 for US$5100.
In October 2001 the Pioneer DVR-A03PK was on sale for $699, and the price
of media had fallen from $50 to $15 per disc.

As mentioned in section (0-2), this FAQ will not be expanding to cover DVD
recorders.  See http://www.dvddemystified.com/dvdfaq.html instead.


Subject: [2-15] What are "jitter" and "jitter correction"?
(1998/04/06)

The first thing to know is that there are two kinds of jitter that relate
to audio CDs.  The usual meaning of "jitter" refers to a time-base error
when digital samples are converted back to an analog signal; see the jitter
article on http://www.digido.com/ for an explanation.  The other form of
"jitter" is used in the context of digital audio extraction from CDs.
This kind of "jitter" causes extracted audio samples to be doubled-up or
skipped entirely.  (Some people will correctly point out that the latter
usage is an abuse of the term "jitter", but we seem to be stuck with it.)

"Jitter correction", in both senses of the word, is the process of
compensating for jitter and restoring the audio to its intended form.  This
section is concerned with the (incorrect use of) "jitter" in the context of
digital audio extraction.

The problem occurs because the Philips CD specification doesn't require
block-accurate addressing.  While the audio data is being fed into a buffer
(a FIFO whose high- and low-water marks control the spindle speed), the
address information for audio blocks is pulled out of the subcode channel
and fed into a different part of the controller.  Because the data and
address information are disconnected, the CD player is unable to identify
the exact start of each block.  The inaccuracy is small, but if the system
doing the extraction has to stop, write data to disk, and then go back to
where it left off, it won't be able to seek to the exact same position.  As
a result, the extraction process will restart a few samples early or late,
resulting in doubled or omitted samples.  These glitches often sound like
tiny repeating clicks during playback.

On a CD-ROM, the blocks have a 12-byte sync pattern in the header, as well
as a copy of the block's address.  It's possible to identify the start of a
block and get the block's address by watching the data FIFO alone.  This is
why it's so much easier to pull single blocks off of a CD-ROM.

With most CD-ROM drives that support digital audio extraction, you can get
jitter-free audio by using a program that extracts the entire track all at
once.  The problem with this method is that if the hard drive being written
to can't keep up, some of the samples will be dropped.  (This is similar to
a CD-R buffer underrun, but since the output buffer used during DAE is much
smaller than a CD-R's input buffer, the problem is magnified.)

Most newer drives (as well as nearly every model Plextor ever made) are
based on an architecture that enables them to accurately detect the start
of a block.

An approach that has produced good results is to do jitter correction in
software.  This involves performing overlapping reads, and then sliding the
data around to find overlaps at the edges.  Most DAE programs will perform
jitter correction.


Subject: [2-16] Where can I learn more about the history of CD and CD-R?
(2002/12/02)

Some information about "the goode olde days" can be found in Robert
Starrett's "The History of CD-R" article, currently available from
http://www.roxio.com/en/support/cdr/historycdr.html.

The first CD player was available in Japanese stores on October 1, 1982.
CD-Recordable technology wasn't introduced until 1988.  For a timeline,
see http://www.oneoffcd.com/info/historycd.cfm.

Back in the late 1980s, CD recorders cost thousands of dollars, and were
part of systems the size of a washing machine.  Disks cost US$100.00 each.

Things started to get better in 1995, when Yamaha released the CDR100
(the first 4x recorder) for a mere US$5000.00.  In September of 1995,
HP released the 4020i (a 2x recorder based on the Philips CDD2000) for
just under US$1000.00.  Media was down to about US$8.00, though 80-minute
discs were extremely rare and expensive (US$40.00 each, if you could find
them at all).


Subject: [2-17] Why don't audio CDs use error correction?
(2007/08/08)

Actually, they do.  It is true that audio CDs use all 2352 bytes per block
for sound samples, while CD-ROMs use only 2048 bytes per block, with most
of the rest going to ECC (Error Correcting Code) data.  The error
correction that keeps your CDs sounding the way they're supposed to, even
when scratched or dirty, is applied at a lower level.  So while there
isn't as much protection on an audio CD as there is on a CD-ROM, there's
still enough to provide perfect or near-perfect sound quality under
adverse conditions.

All of the data written to a CD uses CIRC (Cross-Interleaved Reed-Solomon
Code) encoding.  Every CD has two layers of error correction, called C1 and
C2.  C1 corrects bit errors at the lowest level, C2 applies to bytes in a
frame (24 bytes per frame, 98 frames per sector).  In addition, the data is
interleaved and spread over a large arc.  (This is why you should always
clean CDs from the center out, not in a circular motion.  A circular
scratch causes multiple errors within a frame, while a radial scratch
distributes the errors across multiple frames.)

If there are too many errors, the CD player will interpolate samples to get
a reasonable value.  This way you don't get nasty clicks and pops in your
music, even if the CD is dirty and the errors are uncorrectable.
Interpolating adjacent data bytes on a CD-ROM wouldn't work very well, so
the data is returned without the interpolation.  The second level of
ECC and EDC (Error Detection Codes) works to make sure your CD-ROM
stays readable with even more errors.

It should be noted that not all CD players are created equal.  There are
different strategies for decoding CIRC, some better than others.

Some CD-ROM drives can report the number of uncorrected C2 errors back
to the application.  This allows an audio extraction application to
guarantee that the extracted audio matches the original.


See http://web.archive.org/web/20031211151723/http://www.cdpage.com/dstuff/BobDana296.html
for an overview of error correction from the perspective of media
testing.  If you really want to get into the gory technical details,
there used to be a good page at
http://www.ee.washington.edu/conselec/CE/kuhn/cdmulti/95x7/iec908.htm.


Subject: [2-18] How does CD-R compare to MiniDisc?
(2000/08/08)

MiniDiscs, or MDs, are small (64mm) discs that hold about 140MB of data or
160MB of audio.  By using sophisticated compression techniques they are
able to compress audio by a 5:1 ratio, allowing a capacity of 74 minutes
with little or no audible difference in quality.  As with CD recorders,
there are MD recorders that connect to your computer and MD recorders that
connect to your stereo.

There are stamped MDs that are similar to CDs in construction, and
rewritable MDs that use magneto-optical technology.  Audio MD recorders
are generally more convenient than stand-alone audio CD recorders, because
the playback mechanism allows a more flexible layout of audio data, so it's
possible to delete a track from the middle of the MD and then write a
longer one that is recorded in different places across the disc.  The
current generation of MD technology is unlikely to replace CD-R or DAT,
however, because the lossy compression employed is disdained by audio
purists.  MD is more often positioned as a replacement for analog cassette
tape, which it matches in portability and recordability, and surpasses in
durability and its ability to perform random accesses.

Computer-based MD recorders can write data, but may not be able to record
audio.  Check the specifications carefully.

A wealth of information is available from http://www.minidisc.org/.  If you
want to transfer CD to MD or MD to CD-R, check there for more information.
(It used to be item #37 in the FAQ, but doesn't seem to be now.)


Subject: [2-19] What does finalizing (and closing and fixating) do?
(2002/05/26)

A disc that you can add data to is "open".  All data is written into the
current session.  When you have finished writing, you close the session.
If you want to make a multisession disc, you open a new session at the same
time.  If you don't open a new session then, you can't open one later,
which means that it's impossible to add more data to the CD-R.  The entire
disc is considered "closed".

The process of changing a session from "open" to "closed" is called
"finalizing", "fixating", or just plain "closing" the session.  When you
close the last session, you have finalized, fixated, or closed the disc.

A single-session disc has three basic regions: the lead-in, which has the
Table of Contents (or TOC); the program area, with the data and/or audio
tracks; and the lead-out, which is filled with zeroes and provides padding
at the end of the disc.  An "open" single-session disc doesn't yet have
the lead-in or lead-out written.

If you write data to a disc and leave the session open, the TOC -- which
tells the CD player or CD-ROM drive where the tracks are -- is written
into a separate area called the Program Memory Area, or PMA.  CD recorders
are the only devices that know to look at the PMA, which is why you can't
see data in an open session on a standard playback device.  CD players
won't find any audio tracks, and CD-ROM drives won't see a data track.
When the session is finalized, the TOC is written in the lead-in area,
enabling other devices to recognize the disc.

(Something to try: write an audio track to a blank CD, and leave the
session open.  Put the disc in a CD player.  Some players will deny the
existence of the disc, some will spin the disc up to an incredible speed
and won't even brake the spindle when you eject the disc, others will
perform equally random acts.  The TOC is important!)

If you close the current session and open a new one, the lead-in and
lead-out of the current session will be written.  A TOC will be written in
the current lead-in that points to the eventual TOC of the next session.
This process is repeated for every closed session, resulting in a chain of
links from one lead-in area to the next.  Typical audio CD players don't know
about chasing TOC links, so they can only see tracks in the first session.
Your CD-ROM drive, unless it's broken or fairly prehistoric, will know
about multisession discs and will happily return the first session, last
session, or one somewhere in between, depending on what the OS tells it
and what it is capable of.

Some CD-ROM drives, notably certain early NEC models, are finicky about
open sessions, and will gag when they try to read the lead-in from a
still-open session.  They follow the chain of links in the lead-ins of
each session, but when they get to the last, they can't find a valid TOC
and become confused.  Even though these drives support multi-session,
they require that the last session be closed before they will read the
disc successfully.  Fortunately, most drives don't behave this way.

If you use disc-at-once (DAO) recording, the lead-in is written at the
very start of the process, because the contents of the TOC are known ahead
of time.  With most recorders there is no way to specify that more than one
session should be created in DAO mode, so creating a multisession disc with
DAO recording isn't generally possible.  Such discs must be created with
track-at-once (TAO) or session-at-once (SAO) recording.

If you're using certain versions of Windows, the Auto Insert Notification
feature will "discover" the CD-R as soon as the TOC is written.  This can
cause the write process to fail, which is why Windows software automatically
enables and disables AIN as needed.  Otherwise, if recording in track-at-once
mode, it will fail during finalization; in disc-at-once mode, it will fail
near the beginning of the write process.  In both cases, test writes will
succeed, because the TOC doesn't get written during a test pass.

Packet-written discs follow the same rules with regard to open and closed
sessions, which is why they have to be finalized before they can be read on
a CD-ROM drive.  The "Packet Writing - Intermediate" document in the primer
at http://www.mrichter.com/cdr/primer/primer.htm goes into a little more
detail on this subject.  (Some people like to refer to packet writing as
"PAO", for packet-at-once.)

There are gory details beyond what is written here.  For example, the
lead-in on a CD-R actually has a pre-recorded TOC that specifies physical
parameters of the recording layer, such as required laser recording power,
and information about the disc, like how many blocks can be written (the
"ATIP" discussed in section (2-38)).  You don't usually need to worry
about such things though.


Subject: [2-20] How are WAV/AIFF files converted into Red Book CD audio?
(2001/01/25)

There is absolutely nothing special about the audio data encoded on a CD.
The only difference between a "raw" 44.1KHz 16-bit stereo WAV file and CD
audio is the byte ordering.

It isn't necessary to convert a WAV or AIFF file to a special format to
write to a CD, unless you're using a format that your recording software
doesn't recognize.  For example, some software won't record from MP3 files,
or from WAV files that aren't at the correct sampling rate.  Similarly,
you don't have to do anything special to audio extracted from a CD.
It's already in a format that just about anything can understand.

Just put your audio into the correct format -- uncompressed 44.1KHz, 16-bit,
stereo, PCM -- and the software you use to write CDs will do the rest.
All of the fancy error correction and track indexing stuff happens at a
lower level.

Don't get confused by programs (such as Win95 Explorer) that show ".CDA"
files.  This is just a convenient way to display the audio tracks, not
a file format unto itself.  See section (2-36).


Subject: [2-21] What does MultiRead mean?  MultiPlay?
(2001/10/22)

The MultiRead logo indicates that a CD or DVD drive can read all existing
CD formats, including CD-ROM, CD-DA, CD-R and CD-RW.  See the description
at http://www.osta.org/specs/multiread.htm  The presence of this logo on
a CD-ROM drive does *not* mean that the drive can read DVD.

MultiPlay does essentially the same thing, but is meant for consumer CD
and DVD players.  See http://www.osta.org/specs/multiplay.htm.


Subject: [2-22] If recording fails, is the disc usable?
(1998/07/31)

That depends on what was being recorded, how it was being recorded, and
how far along in the process things were.

If it failed while writing the lead-in, before any data was written, the
disc probably isn't usable.  Some drives, notably certain Sony models, have
a "repair disc" option that forcefully closes the current session.  This
would allow you to add extra data in a second session on the disc, but
anything written in the first session will be unavailable.

Failures when finalizing the disc may be correctable.  Sometimes the TOC
gets written before the failure, and the disc can be used as-is.  Sometimes
you can use a "finalize disc" option from a program menu that will do the
trick.  Other times the recorder will refuse to deal with a
partially-finalized disc, and you're stuck.

Failures in the middle of writing result in a CD-ROM that probably isn't
worth trusting.  Some of the data will be there, some won't.  The directory
for the disc may show more files than are actually present, and you won't
know which are actually there until you try to read them.

Audio CDs recorded in disc-at-once mode are a special case.  Because the
TOC is written up front, the disc is readable in a standard CD player even
if the write process doesn't finish.  You will be able to play the tracks
up to the point where the recording failed.

If you were using a packet writing program like DirectCD, the experiences
of people on Usenet suggest that you are either 100% okay or 100% screwed.
The ScanDisk utility included with DirectCD 2.5 may help though.


Subject: [2-23] Why do recorders insert "00" bytes at the start of audio tracks?
(1998/08/14)

This phenomenon is familiar to users who have attempted to extract digital
audio from a CD-R.  Very often the result of copying an audio CD is an
exact copy of the original audio data, but with a few hundred zero bytes
inserted at the front (and a corresponding number lost off the end).  Since
this represents the addition of perhaps 1/100th of a second of silence at
the start of the disc, it's not really noticeable.

The actual number of bytes inserted may very slightly from disc to disc,
but a given recorder usually inserts about the same number.  It's usually
less than one sector (2352 bytes).

According to a message from a Yamaha engineer, the cause of the problem is
the lack of synchronization between the audio data and the subcode
channels, much like the "jitter" described in section (2-15).  The same
data flow problems that make it hard to find the start of a block when
reading also make it hard to write the data and identifying information in
sync.  According to the engineer, no changes to the firmware or drive
electronics can fix the problem.

Making copies of copies of audio CDs would result in a progressively larger
gap, but it's likely to be unnoticeable even after several generations.


Subject: [2-24] How many tracks can I have?  How many files?
(2002/02/11)

You can have up to 99 tracks.  Because the track number is stored as a
two-digit decimal number starting with "01" (BCD encoded, in case you were
wondering), it's not possible to exceed this.

Tracks must be at least 4 seconds long, according to the standard.
In practice, CD recorders have different notions of how short a track can
be, but most recorders will refuse to write a track shorter than one second.

The maximum number of files depends on the filesystem you're using.  For
ISO-9660, you can (in theory) have as many as you want.  In practice,
DOS or Windows will treat the disc internally as a FAT16 filesystem, so
you are limited to about 65,000 files if you want broad compatibility.


Subject: [2-25] Will SCMS prevent me from making copies?
(1999/04/11)

SCMS is the Serial Copy Management System.  The goal is to allow consumers
to make a copy of an original, but not a copy of a copy.  Analog recording
media, such as audio cassettes and VHS video tape, degrades rather quickly
with each successive copy.  Digital media doesn't suffer from the same
degree of generation loss, so the recording industry added a feature that
has the same net effect.

SCMS will affect you if you use consumer-grade audio equipment.
Professional-grade equipment and recorders that connect to your computer
aren't restricted.  See section (5-12) for more about the differences
between these types of devices.

The system works by encoding whether or not the material is protected, and
whether or not the disc is an original.  The encoding is done with a single
bit that is either on, off, or alternating on/off every five frames.  The
value is handled as follows:

 - Unprotected material: copy allowed.  The data written is also marked
   unprotected.
 - Protected material, original disc: copy allowed.  The data written
   will be identified as a duplicate.
 - Protected material, duplicate: copy not allowed.

There are hardware "SCMS strippers", primarily used in conjunction
with a DAT deck, that strip the SCMS bits out of an S/PDIF connection.
Some of these reportedly introduce unacceptable artifacts into the audio.
It's possible to "wash" the audio by converting it to and from analog
format, but again the quality will suffer.

If you're using a consumer audio CD recorder, SCMS will prevent you from
making copies of copies of protected material.  It will not prevent you
from making a copy of an original disc you have purchased, and it won't
stop you from copying unprotected discs.

Related sites:
  http://www.oade.com/tapers/scms1.html
  http://www.mitsuicdrstore.com/SCMS_nh.html
  http://www.xs4all.nl/~jacg/dcc-faq.html


Subject: [2-26] Is a serial number placed on the disc by the recorder?
(2004/07/15)

In general, no, but it appears that some stand-alone consumer audio CD
recorders write one.  The Recorder Unique Identifier (RID) is a 97-bit code
recorded every 100 sectors.  It is composed of a brand name identifier,
a type number, and a drive serial number.  Recorders such as the Philips
CDR870 write the RID to discourage distribution of copyrighted material.

Windows will show something like "Volume Serial Number is 4365-0FED".
There does not appear to be any way to control this.  Some have suggested
that the serial number is generated based on data found on the disc,
similar to the way that audio CDs can (mostly) be uniquely identified by
the number and durations of the tracks.

On floppy disks and hard drives, the "serial number" is generated based
on the date and time when the disk is formatted.  The four bytes are:

 1. month + seconds
 2. day + hundredths of a second
 3. high byte of the year + hours
 4. low byte of the year + minutes

(From www.zdnet.com/pcmag/pctech/content/solutions/uu1508a.htm, which no
longer exists.)


Subject: [2-27] What's a TOC?  How does it differ from a directory?
(2001/08/01)

The TOC (Table Of Contents) identifies the start position and length of the
tracks on a disc.  The TOC is present on all CDs.  If it weren't, the disc
would be unreadable on a CD player or CD-ROM drive.  CD recorders write the
TOC as part of "finalizing the disc.  (Section (2-19) has some more details
about finalizing discs.)

A "directory" is a list of files.  If you're a Mac user, you're probably
used to the term "folder".  It's part of a filesystem, such as the ISO-9660
or HFS filesystem present on most CD-ROMs.  Audio tracks don't have files,
so they don't have directories either.

There's nothing stopping you from writing a FAT16 or Linux ext2 filesystem
directly onto a CD-ROM.  Whether or not you can read such a disc is a
different matter.  (The Linux "mount" command should allow you to mount
just about anything read-only, but Windows may not be so willing.)  The CD
specification defines the TOC, and there are well-defined standards for
certain filesystems, but [AFAIK] nothing in the CD spec requires that you
fill a data track with a certain kind of data.


Subject: [2-28] What's an ISO?  A CIF?  BIN and CUE?  .DAT?
(2005/02/15)

In common use, an "ISO" is a file that contains the complete image of a
disc.  Such files are often used when transferring CD-ROM images over
the Internet.  Depending on who you're talking to, "ISO" may refer to
all disc image files or only certain kinds.

Going by the more restrictive definition, an "ISO" is created by copying an
entire disc, from sector 0 to the end, into a file.  Because the image file
contains "cooked" 2048-byte sectors and nothing else, it isn't possible to
store anything but a single data track in this fashion.  Audio tracks,
mixed-mode discs, CD+G, multisession, and other fancy formats can't be
represented.

To work around this deficiency, software companies developed their own
formats that *could* store diverse formats.  Corel developed CIF, which is
still in use by Roxio's Easy CD Creator.  (What does CIF mean?  Nobody
knows, though "Corel Image Format" is as good a definition as any.)  Jeff
Arnold's CDRWIN created them as "BIN" files, with a separate "cue sheet"
that described the contents.  You can unpack a BIN/CUE combo with
"binchunker", which is now integrated into Fireburner (section (6-1-50)).

A ".DAT" file could be most anything, but usually it's a video file pulled
off of a VideoCD.  A program at http://www.vcdgear.com/ can convert .DAT
to .MPG, and recording programs like Nero can record them directly.

A ".ISO" file that contains an image of an ISO-9660 filesystem can be
manipulated in a number of ways: it can be written to a CD-ROM; mounted
as a device with the Linux "loopback" filesystem (e.g. "mount ./cdimg.iso
/mnt/test -t iso9660 -o loop"); copied to a hard drive partition and
mounted under UNIX; or viewed with WinImage (section (6-2-2)).  There is no
guarantee, however, that a ".ISO" file contains ISO-9660 filesystem data.
And it is quite common to hear people refer to things as "ISO" which aren't.

A ".SUB" file appears to contain subchannel data.  Some programs pass
these around in addition to one of the above formats.

We now have many different file extensions, including ISO, BIN, IMG, CIF,
FCD, NRG, GCD, PO1, C2D, CUE, CIF, CD, and GI.  Smart Projects' IsoBuster,
from http://www.isobuster.com/, can open and manipulate just about any
disc image format.


(The rest of this section is a philosophical rant, and can safely be
skipped.  This is intended to be more illustrative than factual, and any
relation to actual events is strictly coincidental.)

The term "ISO" is ostensibly an abbreviation of "ISO-9660 disc image",
which is itself somewhat suspect.  ISO-9660 is a standard that defines the
filesystem most often used on CD-ROM.  It does not define a disc image
format.  "ISO-9660 filesystem image" would be more appropriate.

When you capture or generate a CD-ROM image, you have to call it
something.  When a CD-ROM was generated from a collection of files into an
ISO-9660 filesystem image, it was written into a file with an extension of
".ISO".  This image file could then be written to a CD-ROM.  As it happens,
the generated image files were no different in structure from the images
that could be extracted from other CD-ROMs, so to keep things simple the
extracted disc images were also called ".ISO".

(Some programs used the more appropriate ".IMG", but unfortunately that was
less common.)

This meant that, whether you extracted a data track from a disc written
with the HFS filesystem or the ISO-9660 filesystem, it was labeled ".ISO".
This makes as much sense as formatting a 1.4MB PC floppy for HFS, creating
an image, and calling it a "FAT12 disk image" because such floppies are
usually formatted with FAT.  It didn't really matter though, because no
matter what was in the file, the software used the same procedure to write
it to CD-R.

As a result of this filename extension convention, any file that contained
a sector-by-sector CD-ROM image was referred to as an "ISO file".  When CD
recorders hit The Big Time and many people started swapping image files
around, the newcomers didn't know that there was a distinction between one
type of disc image and another, and started referring to *any* sort of disc
image as an "ISO".

These days it's not altogether uncommon to see messages about "making an
ISO" of an audio CD, which makes no sense at all.

More trivia: "ISO" refers to the International Organization for
Standardization.  Because the organization's name would have different
abbreviations in different languages ("IOS" in English, "OIN" in French),
they used a word derived from the Greek "isos", meaning "equal".
See http://www.iso.org/.


Subject: [2-29] Why was 74 minutes chosen as the standard length?
(2002/04/18)

The general belief is that it was chosen because the CD designers wanted to
have a format that could hold Beethoven's ninth symphony.  They were trying
to figure out what dimensions to use, and the length of certain performances
settled it.

There are several different versions of the story.  Some say a Polygram (then
part of Philips) artist named Herbert von Karajan wanted his favorite piece
to fit on one disc.  Another claims the wife of the Sony chairman wanted
it to hold her favorite symphony.  An interview in the July 1992 issue of
_CD-ROM Professional_ reports a Mr. Oga at Sony made the defining request.
(This is almost certainly Norio Ohga, who became President and COO of Sony
in 1982 and has been a high-level executive ever since.)

The "urban legends" web site has some interesting articles for anyone
wishing to puruse the matter further.  The relationship of Beethoven's
ninth to the length is noted "believed true" in the alt.folklore.urban FAQ
listing, but no particular variant is endorsed.

http://www.urbanlegends.com/misc/cd/cd_length_skeptical.html
http://www.urbanlegends.com/misc/cd/cd_length_karajan.html
http://www.urbanlegends.com/misc/cd/cd_length_origin.html

Another entry:

http://www.snopes2.com/music/media/cdlength.htm

Searching the net will reveal any number of "very reliable sources" with
sundry variations on the theme.


Subject: [2-30] Why is there a visibly unwritten strip near the CD-R hub?
(1999/12/17)

You haven't closed the session yet.  The lead-in area, which includes the
TOC (section (2-27)), isn't written until the session is closed.  A space
is left for it that is large enough to see.  Read section (2-19) for more
details on what happens when you close a disc.

You will see the narrow unwritten strip if you:

  - write a disc, telling the program to leave the disc and session open.
  - eject a packet-written disc without having closed it in ISO-9660 mode.
  - have a failure during recording in track-at-once mode.

In some cases it's perfectly normal to see this space; it's where the
lead-in area will be written when the session is closed.  It's not
necessarily a sign of failure.

If you use disc-at-once recording, the lead-in area is written right away,
so after a failure you won't see the gap.


Subject: [2-31] What is "BURN-Proof"?  "JustLink"?  "Waste-Proof"?
(2003/02/20)

BURN-Proof (or BurnProof) is an unfortunate abbreviation of "Buffer-Under-RuN
Proof".  The technology allows you to avoid buffer underruns by suspending
and restarting the write process when the recorder's buffer is about to
empty.  (See section (4-1) if you're not familiar with buffer underruns.)

Ideally, the results of interrupted and uninterrupted writes would be
identical.  In practice, there may be a small glitch at the point where
writing was suspended.  Sanyo recommends 4X or higher speed CD-ROM drives
and audio equipment made in 1995 or later for playback.

The general consensus is that these technologies are effective and do
not result in noticeable glitches.

There are several different, competing technologies.  Here's a sample
of what's out there (note that many of the names are trademarked):

  BURN-Proof (Sanyo)
    Buffer-Under-RuN Proof.  The first.  Can restart the laser after a buffer
    underrun.  For details, see http://www.sannet.ne.jp/BURN-Proof/faq/.
  JustLink (Ricoh)
    Can restart the laser after a buffer underrun.  Data gap length is
    less than two microns.  See
    http://www.ricoh.co.jp/cd-r/e-/e_europe/drive/justlink.html.
  Just Link (AOpen)
    Can restart recording after a buffer underrun.  Data gaps are less
    than 2 microns.  http://www.aopen.com/products/cdrw/JustLink.htm.
    Same as Ricoh's JustLink?
  ExacLink (Oak Technology)
    Can restart the laser after a buffer underrun.  See
    http://www.oaktech.com/products/optical/cdrw/exaclink.html.
  ExactLink (Mitsui)
    Appears to be the same as ExacLink.  Mitsui's pages refer to
    "Oak Technology's ExactLink(tm)".
  SMART-BURN (Lite-On IT)
    Smart Monitoring & Adapting Recording Technology for BURNing.  Can
    restart the laser after a buffer underrun, and will reduce the
    recording speed if it thinks the media can't be written safely at
    the requested speed.  See http://www.liteonit.com/english-s-tech-7.htm.
  Waste-Proof (Yamaha)
    Waste-Proof Write Strategy.  Does some extra work to prevent the
    buffer underrun from happening in the first place, but won't save
    you if one actually happens.
  SafeBurn (Yamaha)
    Can restart the laser after a buffer underrun, and will reduce the
    recording speed it if thinks the media can't be written safely at
    the requested speed.  Data gap length is less than one micron.  See
    http://www.yamaha.ca/computer/cp_safeburn.asp.
  Seamless Link (BenQ, Philips)
    Another one.
  SafeLink (Waitec)
    Another one (no details available?).
  Power Burn (Sony)
    And another one.
  FlextraLink (ASUS)
    Yet another one.

All of these are for situations where your computer is unable to send
data to the drive quickly enough to keep the buffer full.  They will not
help you if your computer loses power, your software crashes, your media
is of poor quality, or you smack the drive hard enough to disrupt the
recording process.

Nearly all CD recorders announced in or after 2001 featured some variation
of buffer underrun protection.

Some related technologies:

  Just Speed (AOpen)
    Reduces the record speed if it doesn't think the media can handle it.
    See http://www.aopen.com/products/cdrw/justspeed.htm.  Probably
    implies running OPC (section (5-11)).  Combined with Just Link.
  Smart Speed (BenQ)
    See above; combined with Seamless Link.
  FlextraSpeed (ASUS)
    See above; combined with FlextraLink.

There are usually lots of trademarked names on the specifications, touting
the benefits of SMART-X for audio extraction or the VAS Vibration Absorber
System.  It's unclear whether one manufacturer's implementation is really
any better than the others, or in many cases what they even do.


Subject: [2-32] Can playing CD-Rs in a DVD player hurt the discs?
(2001/12/18)

There appear to be three kinds of DVD players:

 1. Those that can play CD-Rs.
 2. Those that can't play CD-Rs.
 3. Those that damage the discs.

Kind #2 is the most common.  Kind #3 comes with a warning in the manual
(you do read product manuals, right?) that tells you not to play CD-R discs.
It is possible that some players in category #2 are actually in #3 and
just aren't labeled as such, and it may be the case that players in #3
will only damage CD-Rs with specific formulations.

If playing CD-R discs in your DVD player is important, make sure the
player can handle them before you buy a player.  See section (2-13).

It's a little unclear what the player is doing to damage the CD-R media.
The playback laser would have to be operated at a wavelength and intensity
that caused a change in the recording dye layer.

There are no known instances of DVD-ROM drives that damage discs.


Subject: [2-33] Who *really* made this CD-R blank?
(2000/09/03)

Many of the "big name" media manufacturers don't actually make their
own media.  Instead, they buy from other manufacturers and stamp their
logo on the discs.  Generally speaking, this isn't a bad thing, because
the discs were certified good enough that the Big Brand was willing to
put the company name behind the product.

If you have a picky recorder or player, though, it helps to be able to
try several different pieces of media.  If you buy several different
brands, and they're all coming from the same manufacturer, chances are
they'll all behave the same way, and your time and money will be wasted.

So... how do you tell who really made a piece of media?  The short answer
is: you don't.

It's tempting to believe that CD-R media identifier applications (e.g.
section (6-2-9)) will give you the answer you need.  Unfortunately, the
data you get is unreliable at best.  Charles Palmer, from cd-recordable.com,
had this to say about the manufacturer identification:

  "Two components that many users of these programs always take as gospel
  are Media Manufacturer and Dye Data.  These two readings are next to
  worthless.

  The reason for this is that many CD-R manufacturers (like CD-
  Recordable.com) purchase their stampers (the nickel die that all CD-R
  substrates are molded from) from 3rd party sources.  These 3rd party
  sources (either other disc manufacturers, or mastering houses) encode
  the data that these 'Identification' programs read, at the time that
  the original glass master is encoded.  The 'Manufacturer' information
  that is encoded is usually the name of the company that made the
  master.  Since stampers made from that master will be sold to disc
  manufacturers the world over, all of discs that those manufacturers
  produce from those stampers will contain the same 'Manufacturer'
  information.  Information which is obviously quite erroneous and
  irrelevant.  Very seldom will the 'manufacturer' information encoded on
  a CD-R actually tell you anything other than who made the original
  master. [...]

  The second piece of data (the dye type) is also dubious.  Because most
  master/stamper configurations are designed to be matched to specific
  dye types (Phthalocyanine, Cyanine, Azo, Etc), the 'Dye' information
  that is encoded when the master is produced indicates the type of dye
  that the master was designed for.  This of course, does not assure that
  the manufacturer that buys and uses this stamper will be using it with
  the dye that it has been designed for.  It is quite possible that a
  stamper/dye combination is used by a CD-R manufacturer that contradicts
  the 'dye' information encoded on the master.  Therefore that
  information becomes as potentially misleading as the 'Manufacturer'
  data discussed earlier."

The only reliable piece of information in the "ATIP" region is the disc
length.  See section (2-38) for further remarks.


Subject: [2-34] Can I make copies of DTS-encoded CDs?
(2000/12/13)

Yes.  CDs encoded with DTS (Digital Theater Sound) follow the Red Book
standard for the most part.  The chief difference is that the audio is
encoded with DTS rather than 44.1KHz 16-bit stereo PCM.  If you put one
into an audio CD player, it will recognize the tracks and try to play them,
resulting in a hissing noise.

You can copy DTS CDs the way you would any other audio CD.  Attempting
to convert them to MP3 is a bad idea though -- they're already in a
compressed format.

A common way to play DTS-encoded CDs is with a DVD player connected to a
DTS-capable receiver.  The DVD player passes multichannel audio to the
receiver over an S/PDIF connection.  Many DTS CDs are encoded in 5.1
surround sound, which is kinda neat.


Subject: [2-35] Why 44.1KHz?  Why not 48KHz?
(2001/01/05)

The "Red Book" specification for audio CDs chose 44100 samples per second,
where each sample is 16-bit stereo PCM.  PCM is a fine choice for encoding
audio, stereo is widely recognized and supported, and it's very easy to
manipulate data in 16-bit quantities with existing hardware and software.

Why 44100?  Why not make it a round decimal value like 44000, or a round
binary quantity like 44032?  Why not 32KHz or 48KHz?

In general, the human ear can hear tones out to about 20KHz.  According to
a smart fellow named Nyquist, you have to sample at twice that rate to
avoid "aliasing".  Because of imperfections in filtering, you actually
want to be a little above 40KHz.

According to John Watkinson's _The Art of Digital Audio_, 2nd edition, page
104, the choice of frequency is an artifact of the equipment used during
early digital audio research.  Storing digital audio on a hard drive was
impractical, because the capacity needed for significant amounts of 1 Mbps
audio was expensive.  Instead, they used video recorders, storing samples
as black and white levels.  If you take the number of 16-bit stereo samples
you can get on a line, and multiply it by the number of recorded lines in
a field and the number of fields per second, you get the sampling rate.
It turned out that both NTSC and PAL formats (the video standards used in
US/Japan and Europe, respectively) could handle a rate of 44100 samples per
second.  This rate was carried over into the definition of the compact disc.

The sampling rate for "professional" audio, 48KHz, was chosen because it's
an easy multiple of frequencies used for other common formats, e.g. 8KHz
for telephones.  It also happens to be fairly difficult to do a good
conversion from 48KHz to 44.1KHz, which makes it harder to, say, copy an
audio CD with a "consumer" DAT deck.  (Well, okay, some consumer DAT
decks can do 44.1KHz now, but initially only "professional" decks could
handle the lower frequency.)

There is relatively little difference in audible quality between 44.1KHz
and 48KHz, since the slight increase in frequency response is outside the
range of human hearing.  Some inaudible tones produce "beats" with audible
tones and thus have a noticeable impact, but the improvement from 44.1 to
48 is marginal at best.


Subject: [2-36] What format are .CDA files in?
(2001/01/25)

Actually, .CDA files aren't really files at all.  Windows shows the tracks
on an audio CD as ".CDA" files for convenience.  For example, you can
create a file association for ".CDA" and invoke an audio CD player when
you double-click on a track.

The tracks themselves are in a format almost identical to a common WAV
or AIFF file.  See section (2-20).


Subject: [2-37] What are DD-R and DD-RW?
(2001/03/15)

DD-R and DD-RW are Sony standards for "double-density" recordable and
rewritable discs.  The discs hold 1.3GB of data, and are relatively
inexpensive, but aren't compatible with current CD or DVD players.  You
can only read the discs in a DD-R/DD-RW drive.

The recorders form a middle ground between CD-R and DVD-R in terms of storage
capacity and price, but the lack of compatibility reduces their usefulness.
On the bright side, the drives are expected to be able to record on CD-R
and CD-RW media.


Subject: [2-38] What's an ATIP?
(2002/12/11)

ATIP is an acronym for Absolute Time In Pregroove.  All CD-R and CD-RW discs
have a pre-cut spiral groove that wobbles slightly.  The groove keeps the
write head tracking properly, and the wobble (sinusoidal with a frequency
of 22.05KHz) provides timing information to the recorder.  The wobble is
frequency-modulated with a +/-1KHz signal, which creates an absolute time
clocking signal, known as the Absolute Time In Pregroove (ATIP).

In the lead-in area, which is at the start of the disc, the ATIP signal
can be read to get some information about the disc.  The only really useful
bit of information is the number of blocks on the disc, which is determined
by the length of the pre-formed groove.

The ATIP signal also holds some information about the disc's construction
and manufacturer, but see section (2-33) for some comments about their
usefulness.  http://www.orangeforum.or.jp/e/reference/index.htm used to
have ATIP information, but the "Disc Identification Method" link is now
password-protected.


Subject: [2-39] What are "ML" discs and devices?
(2002/01/18)

"ML" is short for "MultiLevel".  Devices and media constructed by Calimetrics
(http://www.calimetrics.com/) boast 3x the storage capacity and 3x the
recording speed of conventional CD-R and CD-RW media.

CD technology works by measuring the light reflected from the surface
of the disc.  Traditional discs only have two levels ("pit" and "land"),
ML discs have more than one.  By increasing the effective bit density of
the media, you can write 3x as much data in one revolution of the disc,
improving both the storage capacity and the recording speed.

The technology requires minor changes to existing hardware, and requires
discs optimized for ML recording.  Discs written with ML devices will not
be compatible with existing CD players and CD-ROM drives.  However, ML
recorders are expected to be able to record on CD-R/CD-RW media as well,
so ML support could be a low-cost bonus feature on new drives.

[ Announced in early 2002, this never really materialized as a consumer CD
technology. ]


Subject: [2-40] What's CD-MRW?  Mount Rainier?  EasyWrite?
(2004/04/15)

CD-MRW is the working name for a CD-RW storage format developed by the Mount
Rainier Working Group (http://www.mt-rainier.org/).  The Mount Rainier group
has creating specifications for native OS support of CD-RW and DVD+RW, with
the eventual goal of replacing floppies and similar formats (e.g. Zip disks).

EasyWrite is a marketing logo for Mount Rainier compliant drives.  Drives
may be sold with the logo if they pass compliance and robustness tests.

This standard is being promoted by Compaq, Microsoft, Philips, and Sony.
The web site claims support by "over 40 industry leaders", including OS
vendors and PC OEMs.

What this means to you: 500+MB of reasonably fast storage that doesn't
require long formatting delays or the installation of special software.
Discs created with Mount Rainier appear to organize the data slightly
differently from other UDF solutions, so some compatibility problems exist.


Subject: [2-41] What's Audio Master Quality (AMQ) recording?
(2002/05/08)

Yamaha developed Audio Master Quality Recording to compensate for higher
"jitter" in recorded CDs.  This is not the kind of jitter addressed by
"jitter correction" in CD rippers (2-15).  This is the "jitter" that people
selling fancy stereo equipment talk about.

Jitter is time-base error.  It's not a corruption of the digital '1's and
'0's, it's a distortion of the timing in which the '1's and '0's arrive at
their destination.  This doesn't affect extraction of audio, so you don't
need to worry about this kind of jitter when reading a CD or ripping to MP3.
You do need to worry about it when listening to a CD.

The digital signal is read from a CD via an analog process: bouncing a
laser off of "pits" and "lands" on a CD.  Various factors can prevent
the signals from arriving at the right place at exactly the right time.
High-end CD players can correct these anomalies, but many don't.

AMQ extends the length of the pits and lands on the CD in an attempt to
produce a more stable signal.  This reduces the recordable length of the
CD -- a 74-minute disc only holds 63 -- but produces noticeably improved
audio (says Yamaha).  The process works because CD players automatically
adjust the rotation speed.

Yamaha's explanation: http://www.yamaha.ca/computer/cp_AudioMQR.asp

See also section (4-18-2).


Subject: [2-42] Can I draw pictures on a disc with the recording laser?
(2004/03/09)

If you've ever looked at a recorded CD-R, you've probably noticed that
the recorded and unrecorded areas have a different appearance.  This is
usually visible as a slight change in color.  By controlling the write
laser it's possible to mark the disc in a way that is meaningful to the
human eye rather than to a CD player.  Unfortunately, the level of control
required to do this isn't achievable without firmware support.

In mid-2002, Yamaha announced "DiscT@2" (disc tattoo).  This allows
moderate-resolution (approx. 250dpi) graphics to be drawn in the parts of
the disc that weren't recorded.  Yamaha claims to get 256 shades of color
(green, blue, or whatever color the disc happens to be), though it works
best on dark blue azo discs.  For more details and some pictures, see:

 - http://www.tomshardware.com/storage/02q3/020927/index.html
 - http://www.watch.impress.co.jp/akiba/hotline/20020622/etc_disctat2.html

Yamaha left the consumer CD recording market in February 2003, and the
technology quietly disappeared.

In March 2004, HP announced a different idea: flip the disc over, and burn
a design on the label side.  This requires a modified drive and special
media, but offers the possibility of high-resolution labeling without ink
or adhesive labels.  The technology, dubbed "LightScribe", is described
on http://www.lightscribe.com/.


Subject: [2-43] What are the gory details about how are 1s and 0s encoded?
(2002/12/10)

This section is for people who really want to know what's going on inside.
You absolutely do not need to understand any of this to successfully record
a CD.  You will come away with a greater appreciation for CD players,
and also may better understand how some forms of copy protection function.

The sections are written from the perspective of reading a disc.  Generally
speaking, the process is simply reversed when writing.

I tried to find a balance between not presenting enough information and
presenting too much detail.  My hope is that, when you are done reading
this, you will have a broad understanding of how a CD player turns a lumpy
piece of plastic into music, and will know exactly where to look if you
need further details.  If you want the kind of detail found in a textbook,
there are some good ones listed in section (2-43-6).


Subject: [2-43-1] How does the laser read or write a disc?
(2002/12/10)

CD players use a near-infrared 780nm laser.  The visible light spectrum
is generally considered to be 400nm to 700nm; few people can see light
past 720nm.  (DVD, by contrast, uses a visible red 635nm or 650nm laser.)

The drive shines a laser through the polycarbonate (plastic) on the "bottom"
of the disc.  This bounces off the reflective layer, passes back through the
polycarbonate, and is read by a photosensor in the drive head.  The index
of refraction for polycarbonate is about 1.55, so laser light bends when
it enters, allowing a much finer focus for the laser (from 800um at the
bottom of the polycarbonate down to about 1.7um at the metal surface).
This minimizes the effects of dust and scratches, because the effects
of any surface gunk are reduced as the laser's focus width is reduced.
A 400um-wide piece of dust on the surface of a CD would completely block
a laser focused down to 200um at the surface, but has little effect on a
CD player.

If the photosensor sees a strong beam -- the CD standard requires the
signal strength to be at least 70% when fully reflected -- it knows it's
traveling over a "land".  If it sees a weaker response, it's traveling
over a "pit".  Technically, it's traveling "under" a pit or land, so from
its perspective a "pit" is actually a bump.  The height of the bump is 1/4
of the laser's wavelength when traveling in polycarbonate, so that light
reflected from the bump has a phase difference of one-half wavelength.
The light reflected from the pit and from the surrounding land thus cancel
each other out.  (The geometries are actually such that a "pit" reflects
about 25% of the intensity rather than 0%.  For example, pits are 0.5um
wide, or about 1/3 of the focused width of the laser.)

There are a lot of optical tricks involving polarization of light and the
action of diffraction gratings going on.  For example, the read head uses
a three-beam auto-focus system that keeps the laser properly aligned on
the spiral track and at the correct distance from the bottom of the disc.
(Side note: if adjacent loops of the spiral are too close together -- the
"track pitch" is too small -- the laser tracking can fail.  This is why
90- and 99-minute discs are harder to write and read.)  It's also worth
mentioning that, because light travels more slowly in polycarbonate,
the wavelength of the laser inside the CD is closer to 500nm.

CD-R and CD-RW discs do not have pits and lands.  On CD-R media, the write
laser heats the organic dye to approximately 250 degrees Celsius, causing
it to melt and/or chemically decompose to form a depression or mark in the
recording layer.  The marks create the decreased reflectivity required by the
read laser.  On CD-RW media, the write laser changes the material between
crystalline (25% reflectivity) and amorphous (15% reflectivity) states.
This is done by either heating it above its melting point (500C to 700C)
and letting it cool rapidly to convert it to amorphous form, or heating it
to its transition point (200C) and letting it cool slowly to return it to
the more stable crystalline state.  The lower reflectivity of CD-RW makes
the discs unreadable on most older players.

The rest of this discussion refers to "pits" and "lands", but applies
equally to pressed CDs, CD-Rs, and CD-RWs.


Subject: [2-43-2] How do pits and lands turn into 1s and 0s?  What's EFM?
(2002/12/10)

The pits and lands on a CD do not directly correspond to 1s and 0s.
The start and end of a pit (i.e. the pit edges) each correspond to 1s,
and all other areas -- both in pits and on lands -- correspond to 0s.
The number of zeroes between pit edges is determined through careful timing.
This is an efficient approach that produces an easy to handle electrical
signal (it's NRZI -- NonReturn to Zero Inverted -- which converts easily
to NRZ where 1s are high voltage and 0s are low voltage).

The careful timing is possible because CDs are essentially self-clocking.
Suppose you have a clock that ticks once per second.  Plug your ears and
count seconds to yourself, trying to keep the same pace as the clock.
After ten seconds, unplug your ears.  If you've drifted slightly, you can
readjust to the clock without worrying that you've too far off.  You might
be missing the beat by a quarter of a second, but you can adjust forward
or backward a fraction of a second and still be sure that both you and
the clock got to 10 seconds at about the same time.  Now try the same
experiment for 10 minutes.  When you unplug your ears you can get back
in sync with the clock's timing, but unless you have a very good internal
timer it's unlikely you will reach 10 minutes on the same tick.  With your
ears plugged for so long, you are likely to be off by several seconds.

CDs work the same way.  Every pit edge represents an audible clock tick,
while the insides of pits and lands represent inaudible ticks.  If a pit
or land is too long, the drive's clock will drift too far and possibly
get out of sync.  (This is why "blank" recordable discs aren't entirely
blank: they have a pre-cut spiral groove with a "wobble" that the recorder
can use as a timing signal.  A clock accurate enough to produce a stable,
reliable signal at these frequencies is too expensive to incorporate into
a cheap consumer product.  The 22.05KHz wobble is frequency-modulated by
+/-1KHz to create the ATIP signal that, in the lead-in area, holds some
bits of information about the disc.)

To guarantee pits of specific lengths, the CD standard requires that
there are at least 2 and at most 10 zeroes between every 1.  This is
achieved by converting every 8-bit byte into a 14-bit value, a process
called Eight to Fourteen Modulation (EFM).

The shortest possible pit (or land) thus represents 3 EFM bits (100),
and the longest 11 EFM bits (10000000000).  If a single bit requires time
T to pass under the read head, then pits of these lengths can be referred
to as 3T pits and 11T pits.  If after seeking to a new location, the drive
sees a pit shorter than 3T or longer than 11T, then it immediately knows
that the disc is not spinning at the rate it was expecting, and can make
appropriate adjustments.

Between each 14-bit EFM word there are 3 "merging bits".  Because CDs aren't
allowed to have runs shorter than 3T or longer than 11T, it is sometimes
necessary to follow an EFM code with a 1 or 0.  Suppose, for example, that
an EFM code ending in 1 were immediately followed by an EFM code starting
with 1.  The merging bits also serve to prevent the frame synchronization
pattern from appearing where it isn't supposed to (see next section).

If there is more than one possible arrangement of merging bits that satisfy
the restrictions for run length and sync pattern, then a pattern is chosen
that minimizes the low-frequency components of the signal.  This is done by
minimizing the Digital Sum Value (DSV), computed by adding one to a counter
for every T after a transition to a land, and subtracting one for every
T after a transition to a pit.  Adding a 1 to the merging bits inverts
the signal by causing a transition from a pit to a land or vice-versa.
Minimizing the DSV is important because low-frequency signals can interfere
with the operation of tracking and focusing servos.

With EFM there are more bits to encode, but the highest frequency
possible in the output signal is decreased.  The ratio of the number
of bits transmitted to the number of transitions on the medium is high,
making this an efficient way to store the data while still being able to
recover the clock.  It's also worth noting that a 3T pit is 0.833um long,
while the laser spot is just over twice that length at 1.7um.  If 2T or
1T pits were allowed, the laser would have a hard time detecting them.
This is why it's important that transitions not occur too frequently:
the laser is good at computing the time between transitions, but isn't
so good at noticing transitions if they follow each other too quickly.
Making the transitions more obvious requires making the pits and lands
longer, which reduces the amount of data that will fit on the disc.
(See the description of AMQ in section (2-41).)


Subject: [2-43-3] What's a frame?  CIRC encoding?  How does ECC work?
(2004/05/20)

EFM encoding is applied to a series of bytes called a "frame".  Some
sources -- including the SCSI-3 MMC specification -- refer to a CD sector
as a "frame", but that's incorrect usage.  A frame holds 24 bytes of user
data, 1 byte of subcode data, and 8 bytes of parity (error correction),
for a total of 33 bytes.

When read from the disc, each frame is preceded by a 24-bit synchronization
pattern and 3 merging bits.  The sync data has a unique pattern not
found elsewhere on the disc, and it ensures the read head correctly
finds the start of the frame.  (The pattern is 100000000001000000000010,
three transitions separated by 11T, which can't occur otherwise because
the merging bits are specifically chosen to prevent it.)  If you don't
understand why having a sync field is important, remember that every time
the read head seeks to a new part of the disc or is confused by a scratch,
it has to start reading in the middle of a stream of 1s and 0s and try to
make sense of what it's reading.  Until it sees a synchronization pattern,
it has no idea if it's reading the start or middle of a frame, or even if
it's at the start or middle of an EFM word.

The rest of the 33-byte frame is read as 14-bit EFM values followed by 3
merging bits.  This means there are 588 (24 + 3 + (14+3)*33) "channel bits"
in a frame.  This 588-bit structure is called a "Channel Frame".

Once EFM is decoded and the merging bits discarded, we are left with an
"F3 Frame".  The subcode byte is removed, and the remaining data (now an
"F2 Frame") is passed into the CIRC (Cross-Interleave Reed-Solomon) decoder.
The decoder is an important part of the reason why CDs and CD-ROMs work.

The raw error rate from a CD is around 1 error per 100K to 1 million bits.
That's pretty good, but at 4 million bits per second (588 channel bits
per frame x 98 frames per sector x 75 sectors per second = 4.3218Mbps),
the errors add up quickly.  CIRC encoding takes the 192 bits (24 bytes)
of data and 64 bits (8 bytes) of parity, shuffles it around, and performs
some weird math involving Galois Fields.  The bits are processed by two
error correction stages, referred to as C1 and C2.  The efficacy of the
results can be expressed as a set of error counts.

Errors are noted with a two-digit number that indicates the number of
errors with the first digit and the CIRC decoder stage with the second
digit.  The E11 count indicates the number of single-symbol (correctable)
errors in the C1 decoder.  E21 indicates double-symbol (correctable)
errors in C1, and E31 indicates triple-symbol (uncorrectable at C1)
errors in C1.  The sum of these counts is the Block Error Rate (BLER),
a measure of correctable and uncorrectable errors.  The CD standard
sets the acceptable limit to 220 BLER errors per second, averaged over
a 10-second stretch.

The E12 count indicates the number of single-symbol (correctable) errors
in the C2 decoder.  Because the data is interleaved after the C1 pass, one
E31 error can generate up to 30 E12 errors, so a high error count here is
not problematic.  E22 counts double-symbol (correctable) errors, which are
a bad sign.  The sum of E21 and E22 form a burst error count (BST), which
can be used to identify physical defects on a disc.

Any E32 errors, representing triple-symbol (uncorrectable) errors in the C2
decoder, result in damaged data.  For an audio CD interpolation is performed,
for a CD-ROM the damaged data must be repaired at a higher level.  (This,
incidentally, explains how some forms of audio CD copy protection work.
The CD author introduces deliberate uncorrectable errors to the CD.
An audio player will inaudibly interpolate across them, but a CD-ROM
performing digital audio extraction will simply return the bad bits.)
Some software, e.g. Plextor's PlexTools, refer to E32 errors as "CU errors".

With CIRC, the bit error rate is reduced to one in 10 to 100 billion.  The 24
bytes that comes out of the CIRC decoder are referred to as an "F1 Frame".

It's worth noting that the subcode channels are not CIRC-encoded, and hence
are the least-reliable storage directly accessible to the user.  The EFM
encoding provides some protection against single-bit errors, because only
256 of the 16,384 possible combinations are valid, but without any parity
bits the best the drive can do is tell you that it failed to read the
data correctly.  The Q subcode channel, which can hold vital information
about the disc, has a 16-bit CRC.


Subject: [2-43-4] What's in a sector?
(2002/12/11)

98 frames of 24 bytes are combined to form a 2352-byte sector and 98
bytes of subcode data.  The sector is assembled from F1 Frames, which are
byte-swapped, shuffled, and run through a descrambler.  The purpose of
the scrambler is to reduce the likelihood that regular bit patterns will
induce a large digital sum value.

It should be pointed out that the 2352-byte sector is the smallest unit
most CD-ROM drives will allow software to manipulate.  It's only after all
of the above that low-level CD-ROM operations, like "RAW DAO-96" reads and
writes, begin.  This is why making a "bit-for-bit" copy of a disc is tricky.

A sector on an audio CD holds 2352 bytes of data.  16-bit stereo samples
require 4 bytes per sample, so there's 2352/4 = 588 samples per sector.
At 75 sectors per second, that's 44100 samples per second (44.1KHz).
At this point, the processing for an audio CD is essentially complete.
CD players feed the samples through a DAC (or S/PDIF connector) and
eventually out to the speakers, and send the subcode data to the front
panel controller so it can update the HH:MM counter and track number.

A sector on a CD-ROM holds 2048 bytes of user data, leaving 304 bytes for
other purposes.  Every data sector begins with a 16-byte header:

 - 12-byte sync field (00 ff ff ff ff ff ff ff ff ff ff 00)
 - 3 byte address (minute, second, fraction (1/75th) of a second)
 - 1 byte mode

The sync field and address are important because early CD-ROM drives
weren't able to accurately determine the start of a sector.  The drives
were based on CD players, which just shoved the decoded frames into one
FIFO and the subcode data into another.  The CD-ROM firmware was presented
with a stream of bytes, and expected to make sense of it.  This situation
is also responsible for audio extraction "jitter", discussed at length in
section (2-15).

The mode byte determines what the remaining 2336 bytes in the sector
looks like:

 - Mode 0: null data; serves no practical purpose for CD recording
 - Mode 1: the typical CD-ROM layout
   - 2048 bytes of user data
   - 4 bytes of EDC (Error Detection Code, a 32-bit CRC)
   - 8 bytes of reserved space, set to zeros
   - 172 bytes of "P" parity
   - 104 bytes of "Q" parity
 - Mode 2: 2336 bytes of user data, usually used for CD-ROM/XA (see below)

The Mode 1 CD-ROM ECC is independent of and in addition to the CIRC encoding.
It uses a Reed-Solomon Product Code (RSPC) to achieve a combined error
rate of 1 error per 1e15 (quadrillion) bits.

CD-ROM/XA (eXtended Architecture) Mode 2 extends the definition of a Mode
2 CD-ROM.  Form 1 looks like a slight rearrangement of a Mode 1 sector,
with the 8 bytes of space moved ahead of the user data and filled with
a sub-header.  Form 2, intended for compressed audio/video data, has the
8-byte sub-header, 2324 bytes of data, and an optional 4-byte EDC code.
The sub-header contains some channel and data type flags.

A CD session must be written in a single mode, but the XA spec allows the
form to change.  Using CD-ROM/XA Mode 2 allows you to choose between extended
error correction and increased data capacity, and also change your mind
several times in a single track.


Subject: [2-43-5] What's in a subcode channel?
(2002/12/11)

There are 8 subcode channels, labeled P,Q,R,S,T,U,V,W, or sometimes "P-W"
for short.  (The ECMA-130 standard refers to subcode bytes as "Control
bytes".)  Every frame contains one byte of subcode data, and each byte holds
1 bit of P, 1 of Q, and so on.  The bytes from 98 consecutive frames are
combined to form a subcode "section".  The first two bits in each channel
are used for synchronization, leaving 96 bits of useful data per channel
(which is where RAW DAO-96 gets its name).

The P and Q channels are defined by the CD audio standard.  (They are
unrelated to the P and Q parity fields.)  The P channel can be used to
find the start of a track, but in practice most devices use the more
sophisticated Q channel.  Q contains four chunks of information: control
(4 bits), address (4 bits), Q data (72 bits), and an EDC (16-bit CRC).

The control bits determine whether the track holds audio or data, the number
of audio channels (stereo or quadraphonic), and specifies the Digital Copy
Permitted and Pre-emphasis flags.  The address bits determine the format
of the Q data section.  Address mode 1 holds information about tracks,
mode 2 holds a catalog number (such as a UPC code, constant for an entire
disc), and mode 3 contains the ISRC (International Standard Recording Code,
constant for a given track but may change with each track).

A disc has three main regions: the lead-in area, the program area, and the
lead-out area.  Subcode Q mode 1 data in the lead-in is used to hold the
table of contents (TOC) for the disc.  The TOC is repeated continuously in
the lead-in area in case of damage (remember, no CIRC encoding on subcode
channels).  In the program and lead-out area, mode 1 contains track numbers,
index numbers, time within the current track, and absolute time.  Index 0
marks the start of a pregap (pause) before the audio in a track begins,
index 1 marks the start of the music, and indexes 2 through 99 are usually
not set but can be added if desired.

The ability to specify track and index markers when writing a Red Book
audio CD is often referred to as "PQ editing" because that information is
contained in the P and Q subcodes.

Subcode channels R through W are not defined by the CD standard, except
to say that they should be set entirely to zero if not used.  They're
currently used for CD+G (e.g. Karaoke) discs, CD-Text, and some forms of
copy protection.

It is interesting to note that, while bytes from 98 consecutive frames are
used to create a subcode "section", those frames don't have to be from a
single sector.  It's possible for a subcode section to start in one sector
and end in the next.


Subject: [2-43-6] I want even more details
(2004/02/17)

Sheesh.

An excellent reference for is Ken Pohlmann's mammoth _Principles of Digital
Audio, 4th edition_ (ISBN 0-07-134819-0), especially chapter 9 (on compact
discs) and chapter 5 (on error correction).  If you want something a little
slimmer, try his older _The Compact Disc Handbook, 2nd edition_, 1992
(ISBN 0-89579-300-8).

Another good book is _The Art of Digital Audio_, 2nd edition, by John
Watkinson, Focal Press, 1994 (ISBN 0-240-51320-7).

Prof. Kelin J Kuhn used to have some very good information on the
University of Washington web site, but it's gone now, and they're not
available on archive.org.  For historical reference, the original info:
http://www.ee.washington.edu/conselec/CE/kuhn/cdmulti/cdhome.htm has a
number of interesting pages.  In particular, there's a good page about CIRC
on http://www.ee.washington.edu/conselec/CE/kuhn/cdmulti/95x7/iec908.htm,
and http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio/95x6.htm has a
nice explanation of disc construction and optics, especially the three-beam
autofocus.

The page at http://www.tc.umn.edu/~erick205/Papers/paper.html provides
some background information on sampling, aliasing, dither, DACs, and other
relevant topics.

You can get a copy of ECMA-130 from http://www.ecma-international.org/.
This document describes the format of a CD-ROM, including physical dimensions
and optical characteristics, as well as sector formats and Q-channel specs.
It also features some interesting annexes:

 - Annex A: Error correction encoding by RSPC
 - Annex B: Scramble (a description of the pre-EFM scrambler)
 - Annex C: Error correction encoding by CIRC
 - Annex D: 8-bit to 14-Channel bit conversion (has the full table)
 - Annex E: Merging bits (algorithm for computation)

Standards documents, as a rule, are terse and difficult to understand.
ECMA-130 is actually quite readable, and if you understood the preceding
sections you should have no trouble sorting it out.

If you want source code for the CIRC, RSPC, EDC, and scramble functions,
look for Heiko Eissfeldt's edc_ecc.c (and related files).  The code is
part of Mode2CDMaker, CDRDAO, and possibly others.

If you want an explanation of DSV and the problems associated with it,
read the Philips patent on the sector scrambler (US4603413), or one of
the associated patents on removal of DC content from a digital signal.
The full text of the patent can be found at http://www.uspto.gov/.  In brief:

  "[...] If the frequency of such oscillation is comparatively high,
  during the read operation the decision level for detection of the
  channel bit signals may be rendered inaccurate. As a result, read-out
  of the information will be disturbed to such an extent that even the
  error-correction measures cannot prevent errors. Moreover, the tracking
  system for controlling the read laser which reads the channel bits may
  become incapable of keeping the laser beam accurately positioned on
  the track."

It appears that, when the DC offset in the signal becomes too large, the read
head has trouble "seeing" the disc.  The voltage level in the photodetector
has pegged, so the difference between a pit and a land is unnoticeable.

An article at http://www.digit-life.com/articles2/magia-chisel/index.html
examines why one specific file failed to record properly.  It turns out
that, after passing through the scrambler, a piece of the file has a
section that matches the sector header sync pattern.

For some technical information on how CD-Rs are constructed, look through
the uspto.gov site for relevant patents.  For example, US5348841 describes
"Organic dye-in-polymer (DIP) medium for write-once-read-many (WORM)
optical discs".


Subject: [2-44] Digital is better than analog, right?
(2002/12/30)

Not always.

Digital audio CDs are superior to audio cassettes and 8-track tapes, and
digital video DVDs are superior VHS videotapes.  However, the analog film
shown in a movie theater is superior to DVD, and the analog studio master
tape is better than an audio CD.  The sounds that an Apple II makes are
generated digitally, but you wouldn't want to play your CDs that way.

Some formats are better than others.  The low-cost consumer digital formats
are generally superior to low-cost consumer analog formats (except perhaps
for 35mm film, though that's changing).  This does not mean that "digital"
is better than "analog", though many people have that impression because
the consumer electronics companies are marketing products that way.


Digital has some advantages over analog.  The most significant is the
ability to apply various algorithms to reproduce the original digital signal.
With most forms of analog transmission, reconstructing the original signal
without noise and distortions is difficult.  The flip side is that, with too
much interference, the digital signal becomes unusable.  NTSC televisions
(the kind used in North America and Japan) can display a transmission with
a negative S/N ratio, i.e. there's more noise than signal.  (If you're not
part of the "cable TV" generation, think about a picture that was heavily
snowed, but still decipherable.  It was probably a sporting event.)

Digital also has disadvantages, although many of them can be minimized
through careful system design.  The most fundamental problem is the need
to convert the digital signal back to analog.  Human senses are analog,
so audio has to be converted to voltages that drive speakers, and video
needs to be turned into pixels on a screen.  The human eye is pretty easy
to fool -- update the image quickly enough and the brain will believe the
motion is smooth -- but the ear is more discerning.  Slight changes in
frequency and timing, especially in a stereo signal, can be detected.

Many digital formats are compressed with "lossy" techniques.  Algorithms
like MPEG-2, MP3, DTS, and SDDS remove parts of the music to reduce the
storage size.  The parts removed are usually inaudible, though that depends
on how much is removed and how good your ears are.

The upshot of all this is that it's wise to pay attention to what you're
getting.  Don't assume that a digital format is better just because it's
digital.


Subject: [2-44-1] What is "digital" and "digitization", anyway?
(2003/01/05)

Computers store things in "bits", which can be either 0 or 1.  To store
something in a computer, it must be converted to a series of bits.  The
process is called "digitizing".

You've probably seen an egg slicer.  If you haven't, picture a device
that looks like a book resting flat on table.  Instead of pages it has
an egg-shaped depression, and instead of a front cover it has a frame
with thin wires stretched across it vertically at regular intervals.
You raise the lid, insert the egg, and when you press the lid down the
wires cut the egg into thin, round slices.

It usually helps to hard-boil the egg first.

Suppose we want to digitize an egg so we can make a nifty 3D model and
display it on a computer.   Our slicer has 9 wires, so we could end up
with as many as 10 pieces.  We place the egg into the device and slice it.
Now we measure the height of each piece in centimeters (assume the pieces
are perfectly round), measuring the diameter with calipers and rounding
it to the nearest centimeter.  Each slice could go from 0cm (the egg was
short, so there was no slice) to 5cm (the width of our slicer).

When we're done, we spit out something that looks like this:

 1. 1cm
 2. 2cm
 3. 2cm
 4. 2cm
 5. 3cm
 6. 3cm
 7. 3cm
 8. 2cm
 9. 2cm
 10. 1cm

Your eggs may vary.  Storing a number from 0 to 5 requires 3 digital bits,
so if we know that measurements are always in centimeters, we can store
the height of each slice in 3 bits.  We have ten numbers to store, so we
can hold our egg in a mere 30 bits!

When we try to display our digitized egg on a computer screen, however, we
discover a problem.  The image doesn't look like a smooth egg.  Instead,
it looks like a bunch of stair steps in a vaguely egg-shaped pattern.
The sizes aren't right either: our original egg was actually 3.4cm at its
widest point, but we had to round it down to 3cm.

Suppose we improve our measurements down to the nearest millimeter.  Now,
when we have to round off the measurements, the round-off error is much
smaller.  The results look much better, but holding a value from 0 to
50 requires 6 digital bits instead of 3, so we've doubled our storage
requirements to 60 bits.  What's more, the image still looks stair-steppy.

The stairs happen because each slice has a single height value.  When we go
from slice #7 to slice #8, we abruptly jump from 3cm to 2cm.  The reason our
recreated egg doesn't look smooth is because we didn't really capture the
original, in which each slice varied in height from one edge to the other.
Our digitization could only capture the average height of each slice.

There are a couple of ways to improve this.  The first is to guess at
the shape of the original egg, and draw smooth curves based on the data
we have.  This is called "interpolation".  The other approach is to buy a
new egg slicer with wires that are closer together, so we have more slices,
reducing the size of the jump from one slice to the next.  This is called
"increasing the sampling rate".  If you double the number of slices,
you double the number of bits required to hold the digital version.

If you slice the egg finely and measure it accurately, you can get a
nearly perfect representation of the original.  For example, if we create
slices that are one molecule apart, and measure the height to the nearest
molecule, we will have an extremely accurate picture, not to mention a
seriously huge digital representation.  The tricky part about digitizing
something is to choose the height and thickness of the slices such that
the likeness is very good but the digital size is small.


Subject: [2-44-2] How does this relate to CD-DA?
(2003/01/05)

An audio CD cuts a one-second "egg" of sound into 44100 slices, and
measures the "height" of each slice from 0 to 65535 (16 bits).  It does
this independently for the left and right stereo channels, using a format
called Pulse-Code Modulation, or PCM.  The technical shorthand, which you
may have seen in a sound editor, is "44.1KHz 16-bit stereo PCM".

Measuring the "height" of each slice is called quantizing.  The round-off
error in the measurements is called quantization error.  The problems
associated with the error can be reduced by applying "dither" (low-level
noise).

The reason for the number 44100 is explained in section (2-35).  The choice
of 16 bits is also fairly arbitrary, but extremely convenient on a computer.

There are other problems when digitizing (e.g. aliasing) and when converting
back to analog form (e.g. jitter).  See
http://www.tc.umn.edu/~erick205/Papers/paper.html for an introduction.

Newer audio formats, such as Super Audio CD and DVD-Audio, offer different
sampling rates (up to 96000), quantization (up to 24 bits), and numbers of
channels (e.g. 5.1 surround-sound).


Subject: [2-45] What's a CDR-ROM?  CD-PROM?
(2003/02/28)

The term "CDR-ROM" was coined by Optical Disc Corporation in a February
2003 press release, and refers to a disc with writable and non-writable
components.  Some possible uses include burning a unique serial number on a
full CD-ROM, or providing recordable discs with marketing content (e.g. a
few tracks of audio to which more music can be added).  More information
can be found at http://www.optical-disc.com/.

Eastman Kodak had a similar product, called the "CD-PROM", a few
years earlier.  According to their web site, marketing and sales
of the CD-PROM was discontinued in October 2002.  See the notice on
http://www.kodak.com/US/en/digital/progCDR/.


Subject: [2-46] What's HD-BURN?  GigaRec?
(2003/05/02)

In April 2003, a few companies began announcing technologies that allow you
to store larger quantities of data on standard CD-R media.  Unlike DD-R
and "ML" technology, special discs aren't required.  The capacity and
compatibility is different for each.

  GigaRec (Plextor)
    Increases storage capacity by 40%, allowing up to 1GB on a 700MB disc.
    The discs can be read on some unmodified CD-ROM drives.
    http://www.plextor.com/english/news/press/pr04142003.html
  HD-BURN (Sanyo)
    Doubles disc capacity of an 80-minute disc from 700MB to 1.4GB.  A
    firmware change is required before a drive can read the discs.
    Support for extended-length CD-RW media is planned.
    http://www.digital-sanyo.com/BURN-Proof/HD-BURN/

Does it make sense to use these?  The extra capacity is handy, but data
is only useful if you're able to read it.  Check the compatibility of the
hardware you're going to use to read the discs.


Subject: [2-47] What are C2 errors?  What do they say about disc quality?
(2004/05/20)

When people talk about "C2 errors" they are usually referring to the rate
of uncorrectable errors found on a CD.  For an overview of error correction,
see section (2-17).  For a more detailed look, see section (2-43-3).  These
values are returned by "surface scan" tools.

There are two flavors of C2 errors, and not all drives are capable of
reporting both.  Uncorrectable C2 errors indicate data that has been lost.
On an audio CD the missing sound samples will be smoothed over, and on
a CD-ROM the errors may be corrected by an additional level of error
correction, so the flaws may not be noticeable.  Correctable C2 errors
indicate data that is whole but will be lost if the disc degrades any
futher.  Some applications now differentiate between the two by referring
to uncorrectable C2 as "CU error".

The fewer errors of either kind, the better.  The results you get are the
combination of the writer and the media, and in some cases may be influenced
by the quality of the device used to read the CD.  If performing the same
set of operations on two different brands of discs results in consistently
lower error rates on one brand than the other, you will probably be better
off with the lower-error-rate brand.  It is entirely possible that a
different writer would yield the opposite results, so it's not reasonable
to say that brand X is better than brand Y without performing a rigorous
test with a variety of different recorders.

Some discs are poorly constructed, and may deteriorate faster than others.
For long-term archiving, it may be useful to re-examine discs periodically,
especially if you buy "cheap" discs in bulk.  Having fewer errors today
means little if the disc is unreadable in six weeks.

Performing these tests on a disc recorded with track-at-once recording or
packet writing can result in unexpectedly high error counts, because the
gaps between tracks and packets look like damaged areas.

For drives capable of reporting the errors, you can use Nero CD Speed
(http://www.cdspeed2000.com/) to evaluate the error rate.  For a more
thorough examination, you can buy "CD Inspector", which comes with software
and a slightly modified CD-ROM drive
(http://www.hda.de/english/products/checker/cd-inspector/cd-inspector.html).


Subject: [2-48] What are CD+R and CD+RW?
(2003/11/17)

Simply put, they aren't.

There is no such thing as CD+R or CD+RW.  There are a number of different DVD
formats, including DVD+R and DVD+RW, but so far CDs only have -R and -RW.
CD formats with a '+' in them (except for CD+G, which only defines the
subcode channels of an audio CD) are usually typographical errors.


Subject: [2-49] What's HighMAT?
(2004/07/27)

HighMAT stands for High Performance Media Access Technology.  Co-developed
and supported by Microsoft and Matsushita (Panasonic), it was first announced
in October 2002.  HighMAT defines formats for storing digital media (music,
photos, videos) on CD-R/RW discs and (eventually) writable DVD formats.

While many DVD players now recognize MP3 and JPG files on ISO-9660 discs,
they don't all do things the same way, and may not support all formats.
A HighMAT-compliant player would be able to handle all files on discs
created in HighMAT format.  The end result is that you would be able to
record a disc full of music or pictures in HighMAT format and send it to
anybody with a HighMAT player and know that it will work.

This format has not yet been adopted by most consumer electronics companies,
so it remains to be seen whether this will become a significant feature.

For details, see http://www.highmat.com/.


Subject: [2-50] What's VariRec?
(2004/05/05)

VariRec ("Variable Recording") is a Plextor feature intended to let users
modify the laser power when recording audio CDs.  It only works for audio
CDs recorded at 4x.  The theory is that adjusting the laser power up or down
slightly may result in better-sounding discs for a particular combination
of writer and media.

VariRec II increases the write speed to 8x and allows manual selection of
the "write strategy" as well.

In theory there is no need for such a feature, because drives contain tables
of power levels for known brands of media, and can automatically determine
the correct setting for others.  However, some discs use the wrong media
type information, so manual adjustments can be helpful in some cases.

See section (4-18-2) for information about audio CD quality, and (3-31)
for some notes on recording speeds and power levels.


Subject: [2-51] Will my CDs work on players in other countries?
(2004/09/08)

Yes.  Videos sold on DVD usually have region coding that prevents them from
working on players in other countries.  No such restriction is possible in
CD formats.  Audio CDs, CD-ROMs, and VideoCDs will work equally well in any
part of the world.


Subject: [2-52] Do CD-Rs have deeper pits?  Are "shallow burns" bad?
(2004/12/10)

CD-Rs and CD-RWs don't have "pits" in the same sense as pressed CDs.
If the material were burned away, you'd get a distinct odor from your CD
recorder as the combustion by-products escaped.  If the burned material were
trapped in the CD, it would probably rupture the lacquer coat (converting
solid matter to gaseous form rapidly is commonly known as "exploding").

It's not accurate to describe a recorded CD as having "deep" or "shallow"
pits, because it doesn't have pits at all.  The organic dye or phase-change
film changes state in a way that affects how light is reflected.  The result
in a CD player is the same, though the peak reflectivity may be different.
You will get different results from different read heads though, e.g. DVD
players have trouble reading CD-Rs, but rarely have problems with CD-RWs
and pressed CDs.

Incidentally, it's not desirable to have "deeper" pits in a pressed CD.
The depth of the pit is chosen to cause a 1/2 phase difference in the
reflected light.  If the pit were shallower or deeper, the effect would
be lost.

See section (2-43-1) for more information about the physics of reading a CD.


Subject: [2-53] What's a stacking ring?
(2005/04/24)

The term is used to describe a slight ridge near the hub of standard
CD-R media.  This provides a small amount of separation between discs
stacked on a spindle.  You can tell if your discs have stacking rings by
piling them up and then pressing down on the outside edge.  If the stack
compresses slightly, they have the ring; if they're solid, they don't.

The ring is helpful when feeding discs into automated recorders because
it keeps the discs from sticking to each other.  It can interfere with
hub labels or with printing near the disc hub, so you can often order
the same media with or without the ring.

There may be some benefit to using discs with the ring even if you're just
burning the occasional disc and using standard labels.  The ridge is on
the bottom of the disc, which means if you put the disc down on a table,
most of the bottom surface won't be in direct contact.  This could help
avoid scratches.


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[ continued in part 2 of the FAQ ]