Dual SVF resonance glitch?

Is this glitch in my Dual SVF Shruthi-1 resonance normal ? Both oscillators are off, no Env or LFO modulation on the filter, Resonance at maximum with manual sweep of the frequency. Jus listen to the sample here : https://www.dropbox.com/s/2p4w2rbmpp0hvnx/dual-svf-reso-16bits.aif

Thanks for your help !

Hédi K.

This is expected behaviour.

This is not a problem with the filter design (Ambika, which uses the same filter structure on its SVF voicecard, doesn’t have this problem), but with the fact that the oscillators and control signals are PWM, with a carrier at 40kHz. The interactions between the first harmonic of the PWM carrier and the high frequency components produced by the filter might create aliased frequencies that gets into the audio range. Try unplugging the OSC> pin and you won’t get these. That’s one of the reasons why I don’t recommend people to use PWM.

One solution is to trim the “Offset” parameter of the SVF filter board so that the cutoff frequency cannot get too high. I think it can go as high as 16 or 17kHz without causing this intermodulation.

Thanks Pichenettes for these precise informations :wink:
I was just wondering if I did a mistake or if it was a normal behavior.

Hédi K.