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A good description of some effects from USENET.
Newsgroups: net.music,net.music.synth
Path: decwrl!decvax!bellcore!ulysses!mhuxr!mhuxn!ihnp4!houxm!whuxl!whuxlm!akgua!akguc!codas!peora!ucf-cs!novavax!houligan!dave@smaug
Subject: digital delay and effects, part 2
Posted: 15 May 86 22:51:54 GMT
Organization: Gould Electronics, Ft. Lauderdale, Florida.
Xref: decwrl net.music:6297 net.music.synth:1238
Posted: Thu May 15 18:51:54 1986
Summary: some more about digital delays and effects
Keywords: digital delay effects flanging chorus
Line eater: yes
Sorry it's taken me so long to get around to posting this. This is the
second part of my indeterminate series of articles about digital delay
and effects. In part 1, posted a couple of weeks ago, I explained what
the basic effects are that are possible with a digital delay and how to
get them. In this article, I'll go into more detail about two of
these effects: flanging and chorusing.
Flanging: Flanging is created using the shortest time settings on the
delay, in the .5-10 millisecond range. The effect is not created by
the delay itself, but by the mixing of the delayed signal with the original.
If you were to turn the original (or "dry") signal all the way down and
only listen to the delayed signal, you'd hear little or no effect. Without
going into a lot of heavy mathmetics, what happens when the two signals
are mixed is that it creates a pattern of cancelled frequencies that is
usually referred to as a "comb filter" effect (because if you look at on
a spectrum analyzer, it looks like a hair comb, with lots of downward-
facing spikes). Depending on the delay setting, certain frequencies in
the signal will be heavily filtered out. The thing that makes flanging
special is that the frequencies form a pattern that will "pick out" a
certain frequency, and cancel out not only that frequency but also all
of its harmonics. Most musical instruments produce notes that have
a fundemental, lowest frequency (which is what we perceive as the pitch),
and a mix of harmonic frequencies (which we perceive as the "timbre" or
"tone"). Most ordinary filters only filter out frequencies in a certain
range, so with one of these, you might be able to filter out certain
harmonics and make the instrument sound funny, but you could still hear
it. Comb filters formed by flanging can filter out almost *all* of the
harmonics and totally distort the sound of the note, or even make it
disappear entirely.
For a flange effect to be really interesting, the delay time usually has
to be modulated; that is, the delay time is varied slowly over a range
of a few milliseconds. Most digital delays have a built-in modulation
function to do this. This is nice, but it does tend to get a little
predictable after a while. An interesting variation on this scheme is
to use a delay that allows another device, such as a foot pedal, to vary
the delay setting; in this way, the musician can vary the effect to
follow the part being played, or synchronize the effect with something
else (such as the LFO on a synthesizer). Sometimes, you can get a
neat effect by using no modulation at all; this sounds something like
listening to the world through the inside of a paper-towel roll, and is
in fact called the "cardboard-tube" effect. (If you have the Red 7 album,
listen carefully to the end of the last song on side one, I believe it's
called "On the Way You Go", and you can hear an example of this effect).
Delays usually have a control labeled "regeneration" or something like
that; this control sends some of the output of the delay back to the input,
resulting in a more intense effect. This works well with flanging, as
it intensifies the depth of the notches in the comb-filter pattern.
If you run a signal that contains a large mix of frequencies through
a deep flange (pink noise from a synth or the hiss between stations on FM
works well), the flange will add a definite tone to the signal. By
turning the delay setting up and down, you can actually "play" the
flanger!
(A quick word about "phasers" and phase shifting: Phasers were what the
world used before inexpensive digital delays were invented. They produce
an effect similar to flanging by running the signal through a series of
filters. Any (analog) filter introduces a phase shift, and therefore a
time delay, in its output. By running a signal through a set of filters
that don't actually filter anything (these are called "all-pass" filters),
a comb-filter pattern can be set up. But since the time delay isn't
constant, but varies with frequency, the notches in the comb are not
harmonically related, and so the effect isn't as intense. However,
phasing does have its uses.)
Chorusing: This occurs at slightly longer delay settings, generally
in the 10-16 ms range. At this setting, the comb filter pattern still
exists, but it has moved down into the lower frequencies, and so it isn't
really heard as a flange. Most of the chorus effect is from pitch shifting.
Whenever modulation is used to vary the delay time of a sound, the change
in the sampling rate causes the pitch of the delayed sound to go up or
down. This effect becomes more intense as the delay time range goes up.
In the flanging range, it isn't too significant. In the chorusing range,
it is there, not enough to cause a noticible (to most people) bending of
pitch, but enough so that it starts adding things to the sound. Note
that this pitch bending doesn't depend on the delay time, but in the
*change* in the delay time; if there is no modulation, there is no effect.
Ever wonder how a 12-string guitar gets that rich sound? Anyone who
owns one probably knows that if you tune the strings to perfect intervals,
it doesn't sound as good as it does if you tune one string out of each
pair a little sharp or flat. Chorusing works on the same principle;
by mixing the original sound with the same sound made a little sharp or
flat by delay time modulation, the waveforms are altered in a way that
the human ear perceives as "rich" or "full". The name "chorusing" comes
from the sound of many of the same instrument playing the same part.
Picture a large choir or orchestra string section. Each musician is
playing the same part (well, they are for this example), but since no
two human beings can play *exactly* the same thing at the same pitch
at the same time, there is a little distortion occurring in the
composite sound due to all the pitch interactions. (Why does the
ear perceive this type of distortion as pleasing and some other types
as irritating? Cultural conditioning, maybe. Anybody got any theories?)
Ironically, if all the singers or string players were really amazingly
good musicians and could play in perfect pitch, it probably wouldn't
sound as good.
In addition to the pitch shift, there is still that comb-filter effect
lurking down in the lower frequencies. At this delay setting, the effect
of the comb is to cancel or attentuate most of the lower frequencies, which
makes anything run through a chorus sound top-ended. This is most noticable
on something like bass; I've already mentioned Rush's "YYZ" for an
example. (Geddy Lee went through a period where he used this a lot:
"Hemispheres", "The Trees", "Strangiato", etc.) This combines with the
upper harmonics introduced by the mix of the dry signal with the pitch-
shifted signal to produce that characteristic shrill chorusing sound.
The intensity of the chorusing effect varies with the speed of the modulation,
because the pitch shift depends on the rate of change of the delay time.
If you set the modulation really fast, it will probably destroy the
pitch altogether. Plug this into your tuning meter and watch it go crazy.
(Because chorusing does alter your pitch, you should always turn off any
chorus effects you have on before you tune.) If you turn the modulation
off, the effect largely disappears. Regeneration or feedback doesn't
work so well with chorus, because the multiple pitch changes introduced by
this tend to muddy up the sound.
Next time: digital reverb. This is such a huge topic that it may take
more than one posting. Until then...
"But I don't know how to stop..."
Peter Gabriel
Dave Cornutt
Gould Computer Systems
Ft. Lauderdale, FL
"The opinions expressed herein are not necessarily those of my employer,
not necessarily mine, and probably not necessary."
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