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Originally Posted by Casey  So how do these algorithms get away with breaking this fundamental rule of no allpass filters in parallel? Well, the best answer is to trace every input/output path that places the allpass filters in parallel, and make sure they are spaced out enough (say > 200 msecs) that the sound in each path is decorrelated just by being far enough away in time. Much closer than this and coloration starts to show up in the lower frequencies.
If this is not possible, (and it rarely is) then randomizing the allpass filters is typically used to keep moving the comb shaped group delays so that the ear doesn't pick up a constant coloration. |
But if I look at a loop algorithm in a certain well-known reverb, I see a lot of allpasses that are closer than 200ms. In fact, all of them at most sizes less than a canyon. Somewhere around 13ms at the smallest size setting. But the allpasses on either side of that delay are 8.9ms and 4.7ms - the total of which is about the same as the delay between them. Is it more the ratio of allpass length to delay length that is important here? Or to keep the amount of delay within allpasses of the loop to less than, say, 1/3 of the straight delay time in the loop?
Maybe modest gains and enough delay time as a ratio of the allpass time would prevent the effective paralleling of combs - ie. a delay of 13ms at the end of a 4.7ms allpass would allow just under three passes through the first allpass before hitting the next allpass?
Then what about the 'black hole' reverb (I think it was on an Eventide 8000) which is simply a sequence of something like 32 allpasses in series? It has an interesting sound - a very slow buildup and a decay that is about as fast as the buildup. Very unnatural sounding to me. Almost like a tunnel drive-by except without the doppler effect.