This section started by Andreas Koch.

a) enhancements for 2- or 3-channel sound:

- POPS, polyphonic-pokey-sound by Lee Brilliant (3-channel support with one
  Pokey!); refer to ANALOG #66, november 1988, pages 
  54-60; only 1-2 programs exist for this mod., see: 8.14 stereo-software
  for the Atari;

Lee Brilliant writes: (2006.08.09)
    In reality, the POPS device was only a set of connections to the Atari.
    The internal circuitry (Simple to build) was used ONLY to control the
    volume of the sound and to add amplification to power speakers.  One
    could do without the amplifiers if they have their own.  But the POPs
    did not give just two channel sound, it gave _three_.  My design
    allowed for left, center, and right amplifiers and speakers.  It always
    caused a stir at Atari conventions and user groups. The software I used
    with it was POKEY Player which was commonly available then.  One had to
    get that separately and then modify it slightly to drive the three
    channels separately.  At the time there was a lot of support for that
    program and lots of music for it.

- stereo with two Ataris (and thus 2 Pokey chips); use computer/pokey 1
  for the left channel and computer/pokey 2 for the right; no special
  hardware required for this trick (but specially programmed software!);
  see also: 8.14 stereo-software for the Atari;

- stereo with 2 Pokey chips (in one Atari!); refer to an article written
  by Chuck Steinman (which probably appeared in Atari classic?) on how 
  to upgrade your Atari internally with a second Pokey chip; or ask 
  Freddy Offenga for a deluxe-stereo-version, that uses a PCB instead of 
  the piggy-back method. For a list of software that supports this mod. 
  see also: 8.14 stereo-software for the Atari;

- Stereo-Blaster and Stereo-Phaser by Portronic/AMC-Verlag, these were
  hardware add-ons that connected via the monitor port to the Atari and
  gave you "another" monitor port and 2 cinch connectors to connect to
  the monitor and/or the hifi-system; various small paddles (4-10,
  depending on the model you have) make it possible to change amplitudes,
  frequencies, etc. and thus generate a "pseudo-stereo" sound. These
  add-ons also amplified the sound and thus made quality recordings of
  Atari sounds much easier. Alas, these hardware add-ons were quite
  expensive and thus not many (less than 100) were sold. Therefore no
  special software is required, every A8 sound can be changed or 
  enhanced to "simulated-stereo"...

- Stereo-Blaster-Pro, a hardware add-on by Portronic/AMC-Verlag similar
  to Stereo-Blaster and Stereo-Phaser, but programmable! This add-on had
  only 1 small paddle, to amplify the sound-volume; the stereo-sound 
  could be generated via two simple Poke-Statements, a demo-disk therefore
  was included. Alas, not many items were sold and as far as I know no-one
  else programmed stereo-software for it. See also: 8.14 stereo-software
  for the Atari...

b) other sound enhancements:

- Covox Sound enhancement, originally developed in Poland; digital to analog
  (DAC) converter, uses a PIA and a resistor ladder to produce 8-bit sound
  playback. (A viable way to reproduce the 8-bit samples captured from the
  Alphasys Sample Cartridge.) (see also 8.14)
  
- SID-upgrade, the SID is the standard sound-chip in the C64 computers.
  Some Polish freaks/nerds have found a way to include it into an A8,
  but although I have seen quite a lot of pictures (for example at 
  atariarea.nostalgia.pl) with this mod. and already found 1 or 2 
  programs that detect it (for example System Info 2.x by Draco), I 
  have not yet found any schematics for this upgrade. Anyway, it exists, 
  and with some programming skills it would surely be possible to write 
  programs then, that playback SID sounds on those Ataris which have this 
  upgrade installed... 

- AMY sound-chip, the AMY sound-chip was originally produced by
  Atari and installed into the 65XEM computer. Alas, it was never
  available to the public and only very few prototypes of this 65XEM 
  (maybe less than 10?) do exist. Besides of that RUMORS say, that
  Atari had quite some problems with this sound chip and never finished 
  it completely/successfully. If the rumors are true, then this 
  sound chip provided many more sound channels, more octaves and even 
  more and better sound power than two Pokeys together (for more infos 
  take a look at this URL:
  http://www.atarimuseum.com/computers/8bits/xe/xe_protos/65xem.html )

- guess there are dozens of other sound enhancements, for example sound
  cards (like Adlib, etc.), sound-chips, midi-interfaces, etc. that could
  be attached or converted to the A8; I won't name them all here...(A.K.)