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From: keithd@cadovax.UUCP (Keith Doyle)
Newsgroups: comp.graphics
Subject: Re: Comb filters
Message-ID: <1659@cadovax.UUCP>
Date: Fri, 17-Jul-87 17:06:04 EDT
Article-I.D.: cadovax.1659
Posted: Fri Jul 17 17:06:04 1987
Date-Received: Sat, 18-Jul-87 21:09:37 EDT
References: <8707110357.AA14175@unisoft.UNISOFT> <1854@Shasta.STANFORD.EDU>
Reply-To: keithd@cadovax.UUCP (Keith Doyle)
Organization: Contel Business Systems, Torrance, CA
Lines: 62

Hmm.  Comb filters.


The way I heard it, was that comb filters are a series of notch
filters at specific intervals.  Early audio electronic 'flangers'
actually called 'phasers' were apparently implemented using such
a technique, where the filters are swept up and down in unison.

The term 'flanger' comes from 'flanging' which was an early recording
studio technique, where you would take an audio sound, run it through
two tape recorders, (multi head) and sum the output back into a single
signal again.  By resting a thumb on the reel 'flange', the tape delay
of one deck could be increased slightly causing the 'darth vader' 
doppler-like effect.  Basically, a signal added to itself slightly
delayed, where the delay amount is varying, is what causes the flanging
effect.

I had then heard, that someone ran a spectrum analysis on a tape
flange signal, found that it looked like a moving comb filter, and
thus 'phasers' were born.  It also may have some phase shift effect
I suppose.  This was before digital delays, and phasers were the only
electronic means of approximating the tape-flange effect.  I also heard
that as few as 3 'notches' was enough to notice the effect.  The more
notches the more effect.

The phaser (or comb filter) audio effect is somewhat more subtle than
a flanger, but does tend to produce a leslie-like effect of sending the
sound around the room.

Both phasers and flangers are available as guitar effects devices at
any music store that carries electric guitars.

The old MXR Phase-45 was apparently a 3 notch comb filter, while the
Phase-90 was some number more.  An interesting box that came out 
in the 70's was the Roland Jet-Phaser, which I believe was a series
of bandpass filters, instead of notch filters, giving a pronounced
effect of having about 5 Wah pedals all going together.  THere was a
filter -Q control (not called that, resonance I think) which would 
vary the intensity of the effect.  And of course they all have sweep
speed controls.

If you ever decide to buy a flanger box, make sure it has a position
where you can turn off the sweep and vary the fixed delay manually. 
Most of them can these days, some of the early ones couldn't.  These
very short delays can produce pronounced 'tin-can' effects that I've
found very useful.  I picked up an Ibanez flanger that was pretty cheap,
($50 or so) that also had a useful range control, giving a wide delay
range.  You turn off the sweep by varying the sweep amplitude down to
zero, and then can adjust one of the other controls for the baseline
delay that the sweep works from, allowing you to set a fixed delay at
any point within the total range of the device.

Don't know much about the latest in phasers.  I have an old phase-90
and I don't even use that much.  Wouldn't part with my Jet-Phaser
for the world, and you can't get them anymore.

Oh, and BTW, 'chorus' devices are usually flangers tuned for a bit more
subtle effect, and perhaps optimized for voice 'doubling' effects.

Keith Doyle
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