Let's talk about coloration in reverb caused by allpass filters.
The first thing that any designer learns is that allpass filters are great at increasing echo density when placed in the inner loop of a reverb algorithm. This is often desirable and the reason for using the allpass filter in the first place. Unfortunatly, the next thing he learns is that while an allpass is flat in the frequency spectrum, it has a comb shaped group delay characteristic that causes ringing in the tail because certain freqencies are delayed longer than others. Since the group delay of an allpass filter is comb shaped, it is regular and leads to a prounounced metallic sound in the tail.
OK, I was wrong, the first thing that a designer learns is not to put allpass filters in parallel. Every text says so, right? Why is this? Well since the group delay of an allpass is comb shaped, two differing allpass filters in parallel will cause outputs that have phase relationships that are directly related to the super position of the two group delay comb shapes.
And yet, we see many designs that have allpass filters in parallel. No? Even if the inner loop consists of allpass filters in series, look again. Follow every input path to every output path and often you will find that the inner loop does have many paths that place the allpass filters in parallel.
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.
A method has recently been published by Blesser, which modifies the allpass filter in order to eliminate the ringing tail problem brought about by the comb shaped group delay. At first this seems like a pure win, but in reality there is a price to pay in the coloration of the running reverb. Blesser's method alters the allpass filter coefficients so as to turn the allpass into a comb filter. Essentially adding coloration to the running reverb in order to eliminate metallic sounding tails. This is particularly problematic in that the coloration is constant.
It is also beyond the abilities of typical computing resources to randomize these modified allpass filters given the math outlined in the Blesser publication. So nothing can be done to eliminate the primary coloration caused by parallel (in this case modified) allpass filters.
For years we have been tought to listen for impurities in the reverb tail. It is just as important to listen for unnatural coloration in the running reverb.
-Casey