[sdiy] VCO - sine output - why bother?

[rsdio at audiobanshee.com]

On Aug 28, 2016, at 11:59 PM, Mattias Rickardsson <mr at analogue.org> wrote:
> Den 28 aug. 2016 10:16 em skrev <rsdio at audiobanshee.com>:
> > ... Any input to the VCF will affect the resonant frequency, making tracking problematic. ...
> >
> > There have been many times where I painstakingly adjusted resonance and keyboard tracking on a VCF to get a pure sine wave that was playable, only to have the tracking ruined when I mixed in other waveforms or even noise. Try it out!
> 
> I have the feeling that I've heard and even read about this phenomenon for ladder filters specifically... but that it's less of a problem for state-variable filters. But I might be wrong. What filter topologies have you tried this with?

CEM3340, CEM3372, and, I think, the SSM2040.

I don't think I have any discrete ladder filters.

Seems like it happens with any self-oscillating VCF - just feed it an unrelated frequency from an untuned VCO and the VCF frequency will drift. Perhaps with more resonance the input could be diminished, but I haven't really tried to test this everywhere.

Brian