[sdiy] State Variable Filter Practical Maximum Q

[Scott Gravenhorst]

Antti Huovilainen <ajhuovil at cc.hut.fi> wrote:
>On Sat, 17 Oct 2009, Scott Gravenhorst wrote:
>
>> Thanks Jim, that's sorta what I figured, noise generated by active 
>> components would be something that would supply the energy to kick it 
>> into oscillation given enough gain at high Q settings.  I assume this 
>> also applies to voltage controlled state variable filters.
>
>The "problem" with SVF is that the resonance works by decreasing damping 
>unlike cascaded RC filters (moog & most other 4 poles) or those 
>using bandpass coupled positive feedback (MS-20). Decreasing opamp gain 
>results in decreased damping and thus higher Q.
>You should be able to compensate for this by increasing the damping at 
>high frequencies.

Thanks Antti, that makes sense. 

I suppose I should have made my question a bit more specific - I was curious as to how
an analog SVF performs versus a digital one (assuming high sample rate - say 200KHz)
for audio use.  Generally, can we get higher useful Q from the analog or the digital
version (I use FPGAs for DSP so I can control to a good degree the width of the
arithmetic) for a given frequency?  

As Jim stated, Q enhancement makes such a comparison more complicated, and I seem to
notice Q enhancement in digital SVFs as well (pushing Fc higher, but still below 1/6
SR tends to cause instability at very high Q).  It seems to me that there would be an
advantage in using the digital version for high Q needs since there is no active
device noise factor nor thermal noise, although arithmetic processes can contribute
"noise" or nonlinearity in the form of rounding error etc.  I would think that this
can be fixed or subdued with wider arithmetic.

Am I wanking here?

-- ScottG
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