2nd-order allpass VCFs?

Sean Costello costello at seanet.com
Tue Nov 23 23:06:51 CET 1999

Paul Perry wrote:
> At 11:31 PM 22/11/99 -0800, Sean Costello  wrote:
> >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.
> I think this might be dependant on what kind of material is running into
> the filter. If the notch is very narrow, it might 'miss' the harmonics of
> the input signal. Guitarists are familiar with this from the wah pedals.

THis is what I have found with my phaser experiments. High Q notches
tend to sound less prominent for phasing effects - the notches only
briefly null out frequencies in a given signal when sweeping. Some nice
effects can be obtained by combining high Q notches with large amounts
of feedback around the allpass chain, although the resultant effects may
not correspond to what would be obtained with the same technique in the
digital realm.

Sean Costello

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