[sdiy] SSM2040 filter question

[Czech Martin]

IMHO time variance is not allowed for Wiener-Voltera.
If you think about it, it makes am exact  convolution approach
impossible. The work I have seen so far was non linear, time
invariant (e.g. telephone speaker or room acoustics).
And I have the impression the higher orders for the taylor
makes things very ugly to compute.

So time variable, nonlinear systems, and even more
systems with hysteresis seem to have no black box
analysis approach.

And sometimes it can be hard to develop a differential
equation.


So there is still a reason for doing analog computing,
errr.. analog synths and circuits.


m.c.

> -----Original Message-----
> From: Antti Huovilainen [mailto:ajhuovil at cc.hut.fi]
> Sent: Mittwoch, 12. November 2003 13:36
> To: Czech Martin
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] SSM2040 filter question
> 
> 
> On Wed, 12 Nov 2003, Czech Martin wrote:
> 
> > Recently there was a discussion about the Synthacon
> > filter. Another structure.
> > If I remember right.
> 
> Does someone have URL for the schematic?
> Also, where can I find schematics for SVF filter designs that 
> don't use
> the linearizing diodes of an OTA.
> 
> > better amplitude response, but this can
> > make phase things worse.
> 
> Or better. The goal is to get total phase shift of 180 
> degrees at cutoff
> frequency for the feedback loop. This includes unit-delay 
> that introduces
> phase shift that is linearly dependant on frequency (180 
> degrees at Fs/2).
> Using a combination of impulse-invariant and other sections 
> can compensate
> for this quite effectively, so that the amount of tuning 
> needed is not too
> much. I don't use any tuning compensation and at 10khz the
> self-oscillation frequency is about one semitone off.
> 
> > For feedback structures this is "poison", because the stability
> > properties of the circuit will be modelled wrong.
> > IMHO the sample rate must be raised so that the phase properties
> > inside the bandwidth of interest are not affected.
> 
> For general case, yes. For first order filter with not-too-severe
> nonlinearity (I'd say almost all practical filter circuits), 
> this is not a
> problem.
> 
> This DOES represent a problem for resonance, but there are 
> techniques to
> compensate for that (that don't care whether the filter is 
> linear or not).
> In most 4-pole filters, resonance happens at the frequency 
> where filter
> phase is 180 degrees. For self oscillation, the total 
> amplification of the
> filter at this frequency must be 0.25. So you need to 
> compensate for both
> tuning (phase not 180 degrees) and feedback amplitude.
> 
> Stilson & Smith's Moogvcf paper deals with this although the approach
> taken is unnecessarily complicated IMO.
> 
> > IMHO the Wiener-Voltera approach allows for a black box analysis
> > of any nonlinear systems, as long as no "hysteresis" is involved.
> > I.e. no ambiguity.
> 
> Does it allow for time-variance?
> I had an argument about this when I did some tube simulation (using my
> "own" method). There the biasing is time-varying and depends on the
> (average) input amplitude.
> 
> Antti
> 
> Give a man a fire, and he'll be warm that day,
> Set him alight and he'll be warm for the rest of his life
> 
>