[sdiy] SSM2040 filter question

[Czech Martin]

Recently there was a discussion about the Synthacon
filter. Another structure.
If I remember right.

Samplerate: IMHO any mapping from s plane to z plane will give some phase
distortion or frequency aliasing. This is simply because infinite
frequencies have to be mapped to -1 on the unit circle.
Some mappings insert additional zeros at -1 for
better amplitude response, but this can
make phase things worse.

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.

Please keep us updated on your simulation efforts.

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.

m.c.



> -----Original Message-----
> From: Antti Huovilainen [mailto:ajhuovil at cc.hut.fi]
> Sent: Mittwoch, 12. November 2003 12:05
> To: Czech Martin
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] SSM2040 filter question
> 
> 
> On Tue, 11 Nov 2003, Czech Martin wrote:
> 
> > given that the internal sample rate is high enough to prevent
> > too much frequency axis distortion (or in other words:
> > small step size, in order to keep time and phase characteristics),
> > is it justified to think of a nonlinear system as two sections:
> > distortion and linear filter?
> 
> No.
> In many (most?) filters, the nonlinearity is inside the 
> filter stage local
> feedback loop. This results in frequency dependent distortion 
> that drops
> off for frequencies below cutoff (disappearing a couple of 
> octaves below
> cutoff) because of the linearising effect of the feedback 
> (the same thing
> that makes op-amp circuits linear).
> 
> Moog filter doesn't have the same type of feedback, but when the
> differential equation is written down, there is still 
> nonlinearity inside
> the equation (not just between stages). An approximation would be to
> ignore the internal nonlinearities and just insert a tanh() 
> to account for
> the input differential amplifier.
> 
> Samplerate mostly affects just aliasing since you can correct for the
> tuning by normal digital filter techniques.
> 
> > In case of the SSM2040 some nonlinear differential equations could
> > perhaps be derived, and the system could be sectionend, because the
> > individual stages do not influence the previous stages via loading.
> 
> This is the approach I've taken. However, I need to do some spice
> simulations since just sticking in the effect of very low collector
> voltage to differential amplifier equation (tanh()) turned out to be
> difficult (too complicated equations to solve on pen and paper in five
> minutes).
> 
> What other interesting sounding filters are there besides Moog, MS20 &
> SSM2040 that have schematics available online?
> 
> > But there is some work be done to solve this with Wiener-Voltera
> > (which I still not understand to full extend).
> 
> I've only seen Volterra used for the case of memoryless 
> nonlinear system
> (distortion) followed linear memory system (filter).
> 
> > What do yu think about all those programs that claim to emulate
> > analog hardware. What about this Moog emulator?
> 
> Most of them are just some pre/post distortion and lots of 
> fancy words.
> Usually when you see some codename like "True Analog 
> Emulation" it means
> the result is not very good, so they need to do some hyping.
> 
> > Is it that the beautifull screen artwork impresses so many people
> > that the not so good approximation of Moog sound is not heard?
> > Is it that very few people own a Moog that could be payed side
> > by side?
> 
> Probably. Someone who has used a Moog can probably weigh in on this.
> I've never actually used any of the analog filters I've 
> modeled (MS20, 4
> stage OTA, Moog).
> 
> 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
> 
>