2nd-order allpass VCFs?

Martin Czech martin.czech at intermetall.de
Tue Nov 23 16:02:15 CET 1999

:::> 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

I must confess that I have no clue how that filter actually works,
but my ears say, that frequency is changed, too ;->

I guess rocking the pedal moves both, Q and F?


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