[sdiy] State Var Filter with Nonlinear Feedback

[harry]

Actually we DO use nonlinear feedback in the SVF...
(well...some svf)

The Oberheim SEM filter has nonlinear feedback as
a resonance limiter, to stop Q from getting way out of
hand. I'm doing the same to a VCF1e from EFM...

Its interesting how max feedback in a SVF gives
the low resonance condition, just the opposite of
the ladder filter.

Did you try deliberatly clipping various stages ???

H^) hary

Scott Gravenhorst wrote:

> I finally got back to this to see if it's worth
> anything to me and in a word, for me, it is not.
>
> It can add harmonic content, but not across a
> wide bandwidth, rather the distortion kicks in at points
> where the input signal's harmonics correspond to
> that of the filter's Fc setting.  When sweeping
> Fc the harmonics seem to pop in, like there are detents.
> At least that's what it sounds like.
>
> Maybe I need to play with more complex waveforms??  I
> used square, pulse, saw, tri and sine.  All get very
> well screwed up, but not in a way I would call musically
> consistent or useful.  Ie, you might get a cool effect
> on one note, but play a semi up or down and it's gone.
>
> It will also go chaotic at points as well as into
> self oscillation.
>
> I tried several different nonlinearities, but they
> all seemed to have similar sonic characteristics.
>
> At the very least, I can convert it into a plain old
> VC-SVF, so all is not lost.  Scoping around in it,
> it appears to be doing what it should, however
> I'm just rather underwhelmed by it's sound altering
> usefulness.
>
> Schematic is at
>
> http://home1.gte.net/res0658s/Electronics/nonlinear_filter.gif
>
> for anyone interested.
>
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