[sdiy] VCF model/passband gain compensation

[Antti Huovilainen]

On Sat, 20 Dec 2008, Martin Fay wrote:

> I suspect Antti may pick this up, in his Moog filter model a simple FIR is 
> used to introduce a ~1/2 sample delay in the feedback loop for phase 
> correction. I've also successfully used an allpass filter in this role.

I changed to the Stilson & Smith "Compromise" version some four years ago 
(see http://ccrma-www.stanford.edu/~stilti/papers/Welcome.html).

The basic idea is to add a two tap FIR filter between each stage. This 
gives more constant resonance and higher max cutoff. For four stages the 
solution works remarkably well. It should work or higher number of stages 
too, but you will need to find the FIR coefficients with some iterative 
search. For four stages they are [1/1.3 0.3/1.3].

I've put some pseudocode here: http://users.tkk.fi/~ajhuovil/moog.txt

Antti

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