2nd-order allpass VCFs?

[Sean Costello]

Hi everybody:

After posting this, I have had some input from others, and done some
reading in Electronotes (before I pack them up for the move to the SF
Bay Area). It seems like it is fairly easily to modify a state-variable
filter to produce an allpass output - Bernie Hutchins describes this in
some of the Application Notes. However, now I am wondering if it is
really necessary to create allpass filters out of the state-variable
filters in order to get the precise control over the notches. Perhaps
mixing the bandpass outputs of several state-variable filters in
parallel, and subtracting this from the original input signal, will
produce similar results. 

For the "parallel-bandpass filters subtracted from input signal"
technique to produce really nice notches, I have the feeling that you
would need filters that can achieve fairly LOW values of Q, as lower Q
values in the allpass filters tend to produce more prominent sounding
notches. The state-variable filter I programmed in Csound has problems
with values of Q that are less than 1, but I will give this a try with
this filter and other bandpass filters when I have time (which may not
be until the next millenium, in all honesty). Anyone here experimented
with this, especially with low-Q bandpass filters?  Also, would a global
feedback path work to accenuate the effect, as it does with cascaded
2nd-order allpass filters?

Thanks,

Sean Costello

Sean Costello wrote:
> 
> 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