[sdiy] VCO - sine output - why bother?

[Ian Fritz]

On 8/29/2016 10:38 PM, rsdio at audiobanshee.com wrote:
>
> 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
>
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I believe this is to be expected on principle.  The amplitude limiting 
is a non-linear function, so you really cannot think of this as a linear 
system, where simple superpositioning would apply.

Ian

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