From: META000 <META@UNB.CA>
Subject: Re: Flanger pedal help
Organization: The University of New Brunswick
Date: Sun, 20 Dec 1992 21:03:50 GMT

Your basic generic flanger/chorus explanation:

Both flangers and chorus units work by delaying the original signal
and recombining the delayed and real-time version.  The major
differences between them tend to stem from the psychoacoustic
consequences of different delay lengths, and the processing
consequencesof sound sampling, and of varying the amount of time
delay.

First, the controls, then the implications:

1) INITIAL - this sets the basic amount of time that the delayed
portionis delayed by.  Generally this is a minimum time to which
additional delay is added.

2) RATE - this sets the rate at which the time delay is increased
and decreased cyclically.

3) DEPTH - this can mean different things in different units, but it
generally means the amountof additional time delay added to the
amount set by the INITIAL setting.  So, for example, a given unit
may provide between 1 and 16 msec of delay.  If the INITIAL control
is set for halfway up and so is the DEPTH control, the time delay
might vary from a minimum of, say 4 msec, to a maximum of, say, 10
msec.  Generally, flanger/chorus/ and phase-shifter units are
designed with a low frequency oscillator that generates a sweeping
signal of a fixed level, butvariable rate.  The RATE control sets
howfast it sweeps, and the DEPTH control determines how much of that
sweeping signal is sent tothe ciorcuitry that generates and varies
the time delay.

4) MIX- this determines the amount of delayedvs. real-time signal.
Depending on the unit, this can either vary the levels of both
delayed and real-time signals, or just the level of the delayed
portion.  In general, the most intense effects are obtainedwith
equal parts delayed and real-time.  If you have a unit with a single
control that pans between real-time and delay signal, you ca
getdelay only, which produces some interesting effects itself.  On
some units, if the DEPTH control goes by another name (e.g., WIDTH,
or MODULATION), then the DEPTH control might actually be a MIX
control.

5) RESONANCE/REGENERATION - this determines the amount of delayed
signal that is sent back through the delay circuitry again to be
"re-delayed".  These are not often found in dedicated chorus
devices.

6) INPUT/OUTPUT level controls; self-explanatory.

Okay.  Typically, flanging effects are produced by delaying the
signal somewhere between 1 msec (or less, in better units) and maybe
8-10 msec.Chorus effects are obtained by delaying the signal between
maybe 6-20 msec.  There is an unmistakable overlap between the delay
ranges each covers, which is why the psychoacoustic aspects are
important to consider, in conjunction with the electronic aspects.
When a delayed version of a signal is recombined with the original, IF
the original hasn't changed much in that brief time interval, what
comesout of the delay line will be out of phase, and result in
cancellation. This cancellation, howver, will depend on the amount
of delay and the signal being fed into it.  For instance, a 500hz
tone takes 2msec to go through its' complete cycle (2 x 500 =
1000msec), and 1msec to through` half its' cycle.  If a 500 hzsignal
is delayed by 1 msec and then mixed together with the original, the
two will cancel because they are 180 degrees out of phase with each
other.  A 100hz signal, on the other hand, delayed by the same
amount, will not be completely anti-phase at the delay output, and
will not cancel much. When you feed a complex signal likethat of a
guitar into such a time delay arrangement, you get cancellation at a
number of frequencies which are multiples ofeach other.  So, a 2msec
delay will produce cancellation (notches) at 500, 1000, 2000, 4000,
8000, and 16000 hz.  The spacing of these notches earns them the
term "comb filter".

Okay,back to the flanger/chorus distinction.  When the delay is
short, our attention gets drawn to the filtering effect (i.e.,
flanging), and we tend to notice it even as the delay line increases
the amount of delay up into the range of chorussing.  This would
seem to be due to a few things.  First, at very short delays (e.g.,
1msec or less) there are large portions ofthe audio spectrum with
little or no comb filtering(remember the lowest notch generated by a
1msec delay would be at 1000hz which is reasonably high up in the
world of guitar tones), so as the delay moves from short to long, and
the lowest notch audible moves from high to low (longer delays =
lower notch frequencies), we tend to track it, in tandem with the
notes of whatever is being played.

At much longer delays, however, the entire frequency spectrum is
dotted with notches all over the place (e.g., at 10 msec delay,
there are deep notches/cancellations at 100hz, 200hz, 400hz, 800hz,
1600hz, 3200hz, 6400hz, and 12800hz), so we tend not to notice
changes in the notches as much as other things.  WHATother things?
Well, imagine you were walking10 feet behind someone who was walking
at a steady pace.  In order to catch up occasionally, you would have
to walk very quickly for a bit.  In order to reduce the amount the
signal is delayed, the rate at which information is being spewed out of
the delay line has to increase, and this tends to produce a
noticeable change in the pitch of the sound, not unlike speeding upa
taperecorder.

At very short delays a 50% change in "spew-out rate" will not result
in a discernible change in pitch (e.g., 2msec delay sped up to
1msec), but at longer initial delays, a similar proportion of change
produces a verynoticeable change in pitch.  The bottom line is that
while flangers and chorusses BOTH change pitch and result in
cancellations due to time delay, each attracts your attention to one
more than the other, depending on how they are set.

You'll note that if the change in delay is very gradual, you can
probably tolerate very broad changes in delay and pitch, however,
at fast rates, mostusers run screaming for the depth control to turn
it down.  Quick, dramatic changes in pitch are irritating, and not
unlike Bert Lahr singing "If I was the King of Forest" in the Wizard
of Oz.  If one is operating within "flanging range", the user can
probably tolerate reasonably wide sweeps, even at fast sweep rates.

Regeneration is something which can increase the apparent filtering
effect, and add a tankiness or boxiness to the sound, depending on
whether one positive or negative flanging (another lecture for
another time).  When in the range of chorussing, regeneration tends
to simply clutter up the sound, so most chorus units don't include
it.

Since the chorus effect tends to sound best when even amounts of
delay and real-time are combined, many chorus units also don't
include a mix/depth control, although some do.  For flanging,
however, especially is the unit has a resonance control, signal
mixing is a must, since the listener's attention can easily be
distracted from the melody to the"jet plane" sound, and you don't
always want that to happen.

Do the sounds created have to fall into one form or the other?  Yes
and no.  The way most are designed, yes, since the sweeping is
controlled by a circuit that simply repeats itself without changing.
Were one to replace the usual LFO circuitry with something else that
stayed in the flanging range for a little while, and switched to the
chorus range occasionally, then different effects could be obtained
fromthe same unit.  I emphasize, however, that this would take a lot
of studio equipment to do, or some heavy duty MIDI programming,
rather anathema to the floor box under discussion.

Some floor boxes, however, will let you do that.  The old PHLANGER
from PAiA (circa 1976-1980) had voltage control inputs so you could
alter the initial delay time with a foot pedal, in effect being able
to shift ranges from a flanging range to a chorus-like range with
your feet.

Hope this helps. I've got a mountain of grading to do, so I'll knock
off for now.  If you want more details or explanations, I'll gladly
provide them on request later in the week.

Happy Chanukah, Mark Hammer