Saturday, 24 November 2012

Patch Tips #25 - Filters do Maths

Wave-shaping with state variable multi-mode filters, or how to integrate a square into a saw.

A while ago Josh from Toppobrillo made an aside in a forum thread that his Multifilter could be used to bend squares into saws. Infuriatingly, he never said how. Well, I love a patch challenge …

Send a square to the input of your filter and monitor the lowpass output. Wind the frequency knob back until you get something resembling a triangle. Patch the bandpass output to the CV input and start to raise the resonance. Play with the input level, amount of feedback and resonance until you get something resembling a saw. It won't have a perfectly vertical flank, but it's pretty close.

This patch works because of phase offsets between the lowpass and bandpass outputs. Positive feedback will give you a saw, negative feedback a ramp. If you increase the amount of feedback, you'll be able to generate sub-harmonics of the input frequency. If you send your pitch CV to both the VCO and 1V/O input of your filter, the saws should maintain their shape as the frequency changes.

1 comment:

robert said...

Very nice! Have my diy xr-vcos with sine, triangle and square and was looking for another saw. This is perfect! Thx!