Quote:
Originally Posted by Casey  Hmmm, not sure I see what you mean.
An allpass filter does not have a metallic ring, it is allpass. Allpass filters can lead to a metallic sound because of their regular spectral smear. At the same gain, smaller allpass filters will have less spectral smear, and will cause less of a metallic residue in the tail. |
Allpass is allpass, but this doesn't mean that all of the poles are the same distance from the unit circle. If you have a few poles that are closer to the unit circle than others, it seems like these will ring out longer than the others in the decay. Put feedback around these allpasses, and the poles will get even closer to the unit circle.
As far as allpass filters not having a metallic ring, try creating an allpass delay nested 10 deep, with all the allpass coefficients at 0.7, and total delay at 20 msec, and run an impulse through it. This is an allpass system, but its decay is perceived as VERY metallic.
We may be arguing about semantics here. You say spectral smear, I say ringing poles. The point is that nested allpass filters are tricky to work with.