[sdiy] State Var Filter with Nonlinear Feedback

[Scott Gravenhorst]

harry <harrybissell at prodigy.net> wrote:
>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 ???

No, actually, I was trying NOT to on purpose, mostly 
because I wanted to test what the intentionally inserted
nonlinearity was doing.  But this is more food for thought.
And in fact, my circuit uses the linearity improving diodes.
(heh heh, for a filter with nonlinear feedback...)

>
>H^) hary

Who the heck is 'hary'  ???   (c:

>
>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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>>
>> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
>> -- Linux Rex         | RedWebMail by RedStarWare
>> -- FatMan: home1.GTE.NET/res0658s/TekLab_FatMan_WebSite/index.html
>
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=========================================================  
- Government: The other religion.
- The media's credibility should always be questioned.
- Lambs who lie down with lions are lunch.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.GTE.NET/res0658s/TekLab_FatMan_WebSite/index.html