Delay Lines and Effects
losses whose magnitude is dependent on the fractional length of the delay line.
As the delay length is varied, these variable losses give an amplitude distortion
due to amplitude modulation of the various frequency components. Coupled to
amplitude modulation, there is also phase modulation due to phase nonlinearity
of the interpolator, in both cases of FIR and IIR interpolation.
The terminology used for audio effects is not consistent, as terms such as
flanger, chorus, and phaser are often associated with a large variety of effects,
that can be quite different from each other. A flanger is usually defined as an FIR
comb filter whose delay length is sinusoidally modulated between a minimum
and a maximum value. This has the effect of expanding and contracting the
harmonic series of notches of the frequency response. The name flanger derives
from the old practice, used long ago in the analog recording studios, to alterna-
tively slow down the speed of two tape recorders or two turntables playing the
same music track by pressing a finger on the flanges.
The name phaser is most often reserved for structures similar to the comb
FIR filter, with the difference that the notches are not harmonically distributed.
Orfanidis  proposes to use, instead of the delay line, a bunch of parametric
notch filters such as those presented in sec. 2.2.4. Each notch is controllable in
its frequency position and width. Smith , instead, proposes to use a large
allpass filter instead of the delay line. If this allpass filter is obtained as a cascade
of second-order allpass sections, it becomes possible to control and modulate the
position of any single pole couple, which represent all the single notches of the
overall response. A common feature of flangers and phasers is the relatively
large distance between the notches. Vice versa, if the notches are very dense,
the term chorus is preferred. Orfanidis , suggests to implement a chorus as
a parallel of FIR comb filters, where the delay lengths are randomly modulated
around values that are slightly different from each other. This should simulate
the deviations in time and height that are found in performances of a choir
singing in unison. Vice versa, Dattorro  says that a chorus can be obtained
by same structure used for the flanger, with a difference that the delay lengths
have to be set to larger values than for the flanger. In this way, the notches
are made more dense. For the flanger the suggested nominal delay is 1msec and
for the chorus it is 5msec. If the objective is to recreate the effect of a choir
singing in unison, the fact of having many notches in the spectrum is generally
disliked. Dattorro  proposes a partial solution that makes use of a recursive
allpass filter, where the delay line is read by two pointers, one is kept fixed and
produces the feedback signal, the other is varied to pick up the signal that is fed
directly to the output. In this way, when both the pointers are at the nominal
position, the structure does not introduce any coloration for stationary signals.
A final remark is reserved to the spatialization of these comb-based effects.
In general, flanging, phasing, and chorusing effects can be obtained from two
different time-varying allpass chains, whose outputs feed different loudspeakers.
In this case, sums and subtractions between signals at the different frequencies
happen "on air" in a way dependent from position. Therefore, the spatial sen-
sation is largely due to the different spectral coloration found in different points
of the listening area.