[sdiy] [OT] Analog synths with 2 pole filters

[Scott Gravenhorst]

I might be totally off here, but this sounds like Q enhancement is at least part of
this effect.  I noticed this with digital state variable filters and then read that
it also happens with analog ones.  In my digital filters, I used some arithmetic to
compensate by reducing the feedback as Fc is increased.  In the project where I had
done this, I was not interested in causing oscillation, rather it was to us the
filter as a tonal resonator that could be whacked with an impulse.  Higher notes had
a much longer sinusoid decay and were of noticably higher amplitude than lower
notes.  The compensation helped to mitigate the effect of Q enhancement.

I'm not sure what term I would use to describe Q enhancement, it seems to be part of
the deal with (at least) state variable filters.  For my way of thinking, a
"problem" would be an error in the design or the use of substandard parts and in
such a case a solution would be to correct the error or not use substandard parts. 
This doesn't seem to be the case with a SVF.  It may be that compensation is a
proper path.  Well, I can say that because that's how I fixed it...  (c;


Tom Wiltshire <tom at electricdruid.net> wrote:
>
>On 22 Feb 2013, at 17:35, cheater cheater <cheater00 at gmail.com> wrote:
>
>> Hi Tom,
>> regarding the SSM2164 VCF, why not compensate the feedback as the
>> cutoff CV changes?
>> 
>> On the topic of cutoff-dependent resonance, the WWAYM NWSynth VST
>> plugin had something like this (you could set the resonance make-up
>> via several sliders, like a graphical EQ, and then you had an overall
>> resonance knob). That made its tired old DSP code actually sound
>> interesting.
>> 
>> Cheers,
>> D.
>
>It's a possible solution, but it's compensating for the problem, 
>rather than actually solving the problem. In that respect, it's 
>not such a good solution. 
>
>Still, if the filter was being driven by a microprocessor which 
>had control over the "resonance" (damping) that's probably 
>exactly the approach I'd take - stick a compensation table in the 
>code. Dirty, but simple and effective. 
>
>T.
>
>
>
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-- ScottG
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