[sdiy] Analog Modeling, with a computer!

[Antti Huovilainen]

On Wed, 14 Sep 2005, Sean Costello wrote:

> One thing that is important to realize: Cheating is OK when it comes to
> computer audio. For most of the industry, the goal is to get a good sound,

Certainly, unless all of your marketing is based on touting how every 
single component is truthfully modelled (certain french company).

> but as CHEAPLY as possible. Cheap, in terms of CPU cycles, memory usage,
> etc. The less CPU cycles, the more voices you can get, and/or the cheaper
> the DSP you can use.

In my opinion many authors concentrate too much on computational 
simplicity - many VA synths are still using the original Chamberlin SVF 
filter while computer speed has increased over ten thousand times since 
the original publication.

> - Another takes shortcuts, based on the assumption that the exact physical
> model is not as important as the sonic results. This school uses waveguides
> for physical modelling, and other shortcuts for analog modelling that

The problem I see with this is that the models are not easily extensible 
when changes are made to the modelled system. With "true" physical/circuit 
modelling, you simply copy the changes more-or-less 1:1 to the digital 
model and you're set to go. Of course, being from HUT, I'm biased on this.

> - 1996 - Tim Stilson publishes paper on digital realizations of the Moog
> filter. Discusses root locus, as well as fact that feedback delay needs to
> be accounted for. The paper discusses a lot of different realizations,

The Root-Locus part of the paper always seemed to be completely missing 
the point of the Moog filter (mainly that as long as total loop phase 
shift at cutoff is 180 degrees, you're ok). Either I'm stupid or Tim 
Stilson had a brainfart :). The different realizations are of course 
still useful.

> (polynomial waveshaping used, that is not necessarily tanh based). Thornburg
> publishes paper in 1999 discussing the nonlinearities, but not in the
> specific Moog filter context.

Can you give the reference information for this paper?

> - 2004 - Antti Huovilainen publishes paper describing derivation of Moog
> filter from circuit modelling perspective. Ported over to Csound and
> Supercollider. Excellent sound. Assumes oversampling. Uses multiple
> instances of tanh - 8 per instance of filter (where the filter instance

Actually 5 tanh calculations per sample are enough if you apply trivial 
transformation to the filter structure (which keeps the results identical, 
just a mathematical shortcut).

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova