2nd-order allpass VCFs?

Curtin, Steven D (Steven) sdcurtin at lucent.com
Tue Nov 23 15:28:02 CET 1999


> >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.
> 
What's interesting about about many wah-wah pedals is that they don't vary
the cutoff frequency of the filter, since this would be too expensive.
Instead they vary the Q of the filter, by changing the feedback resistance.
This would seem to be a good use for voltage controlled Q if you're
emulating the sound of a wah-wah pedal using a modular VCF.  I found this
out when looking through the schematics on the web for such classics as the
Crybaby.

Steve C

http://www.emf.org/subscribers/curtin/




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