[sdiy] SSM2040 filter question

[Antti Huovilainen]

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

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Set him alight and he'll be warm for the rest of his life