[sdiy] SSM2040 filter question

[Czech Martin]

A more basic question on simulating nonlinear audio circuits:

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?
In that case a taylor series expansion and the usual digital filter
stuff will do the trick.

OTOH: if the "nonlinearity" and the "filter" interact you have trouble.

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.

If the system can not be sectioned, or even worse, if it is a
black box where only input and output is known, you are in trouble.
But there is some work be done to solve this with Wiener-Voltera
(which I still not understand to full extend).


What do yu think about all those programs that claim to emulate
analog hardware. What about this Moog emulator?
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?

All those plug ins, a friend showed me a "realtime" frequency shifter.
I let him shift a low frequency sine wave and you could
hear the pure sideband rejection...

m.c.



> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of René Schmitz
> Sent: Freitag, 7. November 2003 19:14
> To: Antti Huovilainen
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] SSM2040 filter question
> 
> 
> Hi Antti et al.,
> 
> > I would like someone explain how SSM2040 differs from 4 
> OTAs. To me each
> > section looks identical to CA3080 + darlington follower.
> 
> The difference is that the CA3080 can sink and source current to both 
> rails, while the 2040 gain cell can sink only to -800mV below "GND" 
> (which is 5V in my version). As I explained earlier, this leads to a 
> dynamic nonlinearity.
> 
> > Also, how is the feedback amplitude limited since there is 
> no nonlinearity
> > in the feedback path in that version?
> 
> But, there is one inside the loop and thats all it takes to 
> delimit the 
> amplitude.
> 
> > There are then three possible conclusions:
> > - Input goes close to supply rails and thus there is 
> asymmetric distortion
> >   (unlikely to sound good there).
> > - Darlington pair has highish output impedance, thus 
> distorting the audio.
> > - SSM2040 doesn't clip asymmetrically and sounds good for 
> other reasons.
> 
> I'd say, get your self a Spice simulator, and see what happens for 
> yourself.
> 
> Cheers,
>   René
> 
> 
> -- 
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
> 
> 
> 
>