[sdiy] State Var Filter with Nonlinear Feedback

[Scott Gravenhorst]

"Craig Critchley" <craigc at nwlink.com> wrote:
>If your nonlinear element was cubic you'd have a Ueda attractor, right?

Yeah, but so far I'm doing only really really simple stuff, like
inverse parallel diodes.  Once I understand this box with those,
I'll get more adventurous.  Even with simple diodes, this filter
can, under the right conditions, become unstable.

>  so
>its not surprising it turns chaotic at some point... the harmonics popping
>in, are they frequency doublings (octaves)?

Watching the scope it appears to be 'locking' onto integral harmonics.
A square wave input might be converted to what looks like a square
wave which is amplitude modulated with 2 sine cycles for each 1/2 
square cycle.  As you adjust Fc, all of a sudden, 2 sine cycles become 
3, etc.

>
>                ...Craig
>
>----- Original Message -----
>From: "Scott Gravenhorst" <music.maker at gte.net>
>To: <synth-diy at dropmix.xs4all.nl>
>Sent: Wednesday, October 17, 2001 7:16 PM
>Subject: [sdiy] State Var Filter with Nonlinear Feedback
>
>
>> 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.
>>
>> =========================================================
>> - 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
>>
>>
>

=========================================================  
- 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