Fundamental VCF: weird behavior of high pass filter

  1. Make a VCO

  2. Plug the sawtooth output into a VCF

  3. Set the VCF’s frequency, resonance and drive all to min

  4. Plug the VCF’s HPF output into a Scope

  5. Plug the original wave coming from the VCO into the same Scope

  6. Compare!

It seems like with filter frequency at min, the HPF of a moderate frequency wave should look similar to that wave. But no! Try different kind of waves and see. My favorite is what happens to a digital square wave, but all waves are affected, even sines.

Is this known?

(Note that LPF doesn’t have the symmetrical problem. With the filter’s frequency at max, the LPF of a moderate frequency wave is always very similar to that wave.)

Background: I’m trying to match a cello sustain sound by passing a sawtooth wave through a bunch of VCF’s plugged into a mixer. Naively I assumed that with the default VCF settings, LPF+HPF=IN and the whole thing can work as an equalizer. But instead it noticeably reduces volume and cuts high frequencies. I narrowed it down to the HPF problem above. For a cello sound in particular, the frequencies affected by the problem are very noticeable and important.

If you’re talking about Fundamental VCF, yeah, the HPF has been broken for a while. Too busy with other projects to fix it.

Wow, you’re the fastest responding developer I’ve ever seen, and your project is the best looking open source project I’ve ever seen. Thanks for the good work! I’ll try using other filters in the meanwhile, no worries.