[music-dsp] prime number's role in reverb
music-dsp at umminger.com
Sun Jul 20 22:56:01 EDT 2003
----- Original Message -----
From: "James Chandler Jr" <jchandjr at bellsouth.net>
> Regardless of tap delay ratio, the echoes will reinforce at some point
> during the decay tail. If a ratio is something like 1 : 1.1 -- the
> echoes start out close-spaced, diverge, then re-converge again. Same
> deal for any non-integral ratio.
> One could design a non-integral set of multiple tap ratios that "ought
> to" be fat, but in practice the reverb might start fat, quickly evolve
> to thin, and then diverge to fat again, giving a pretty unsatisfactory
> tail characteristic.
> James Chandler Jr.
If two comb filters c1 and c2 have delays d1 and d2, then they phase from
fat to thin and back at the beat frequency 1/d1-1/d2. Similarly comb filter
c3 will phase with c2 at the beat frequency 1/d3 - 1/d2. By choosing
1/d3-1/d2 = 1/d2-1/d1 (i.e. d3 = d1 d2 / (2*d1-d2)), these beat frequencies
can be made equal. Then, in theory, an appropriate predelay before c3 can be
used to set the beats 180 degrees out of phase to make the density remain
This idea can be extended to any number of combs. In practice, however, it
doesn't seem to really work all that well.
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