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

Tom Wiltshire tom at electricdruid.net
Fri Feb 22 22:44:37 CET 2013

Isn't the effect due to there being a single sample delay in the digital version? - this produces a more significant phase shift at higher frequencies, exactly like the similar (but totally unrelated) effect in the analog version. So although it shows up with both types of SVF filters (analog and digital), I wouldn't say it's to do with SVF filters per se, since the causes are different in the two cases.

Still, interesting to hear that you fixed it in the digital case with exactly the solution Damian proposed for the analog case!


On 22 Feb 2013, at 18:55, Scott Gravenhorst <music.maker at gte.net> wrote:

> 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;

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