[sdiy] Analog Modeling, with a computer!

[Antti Huovilainen]

On Wed, 14 Sep 2005, Sean Costello wrote:

> I've heard theories that the certain French company uses feedback FM for its
> oscillators. That would count as cheating for sure.

The spectrum pictures in advertising material suggest BLIT / (min)BLEP 
approach (incidentally, BLEP sound quality is superior to BLIT-SWS but 
that is another discussion).

> Well, put your money where your mouth is, and crank out a digital SSM 2040!
> I'd like to hear it! How about a diode ladder filter?

Will SSM2044 do?-)
I'm working on SSM2040 too but I need to figure out some weird behaviour 
the spice model exhibits.
As for diode ladder, writing the differential equation is tricky but I 
will do it someday soon.

> This is something I meant to ask you about - I had a DFII one-pole with
> embedded tanh() that I used for emulating the clipping stages of a Big Muff,
> and it seems like it would work well in your filter. Presumably this is what
> you are talking about.

No. The results are identical (within roundoff error).
Original:
y1 += g*(tanh(x) - tanh(y1));
y2 += g*(tanh(y1) - tanh(y2));
y3 += g*(tanh(y2) - tanh(y3));
y4 += g*(tanh(y3) - tanh(y4));

Optimized:
y1 += g*(tanh(x) - t1);
t1 = tanh(y1);
y2 += g*(t1 - t2);
t2 = tanh(y2);
y3 += g*(t2 - t3);
t3 = tanh(y3);
y4 += g*(t3 - tanh(y4));

Pretty obvious.

Antti

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