[sdiy] State Var Filter with Nonlinear Feedback

[Grant Richter]

This is why it is called an "attractor" the function is attracted to stable
points which are numeric multiples. Unstable intervals between the attractor
points are the actual chaotic behavior. A good example is a flute, the
"chiff" is chaotic behavior and the tone is when the energy settles into a
stable attractor.

It can actually be difficult to get at the chaotic behavior. Depending on
the coefficients, the attractors may be so strong, the circuit will not
operate in the chaotic mode. A solid tuned pipe has very strong attractors
(tight seal on flute pads) as the pads become leaky, the attractors become
weaker.

> 
> Watching the scope it appears to be 'locking' onto integral harmonics.
> A square wave input might be converted to what looks like a square
> wave which is amplitude modulated with 2 sine cycles for each 1/2
> square cycle.  As you adjust Fc, all of a sudden, 2 sine cycles become
> 3, etc.