2nd-order allpass VCFs?

Sean Costello costello at seanet.com
Mon Nov 22 18:56:57 CET 1999

Hi all:

Has anyone here ever constructed a voltage-controlled 2nd-order allpass
filter? I'm not even sure how this would be done - a modification of the
state-variable filter, perhaps? 

As to why you would want to do such a thing, my experiments with
2nd-order allpass filters in computer music have yielded some wonderful
sounds. The standard phasing sweeps still sound best with cascaded 1st
order filters with overall feedback. However, 2nd-order filters allow
for precise control over the width, depth, and frequency of EACH notch
in the frequency spectrum. Very useful for creating complex phasing
sounds, comb filters that move in and out of harmonic ratios for the
notches, and paradoxical "barberpole" effects.

Anyway, if anyone wants to play with these in the digital realm, version
4.0 of Csound implements both 1st and 2nd order allpass filter chains
(look for phaser1 and phaser2, and email me if my instructions on how to
use these make no sense). I'd love to hear if anyone has experimented
with these filters in the analog realm, as a bank of several of these
cascaded together, with some clever control voltage processing, would
make for some amazing sounds.

Sean Costello

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