Thomas Huber wrote:


> A state-variable-filter - oh yes yes, please, could you code one ?
> I'd love to have another excellent sounding filter like 'moogvcf'.
> And as far as I know, the state variable filter can be used as
> lowpass, bandpass, highpass and notch (band-reject) at the same time,
> depending on where you take the output (in the analog circuit, I have
> no indea how it is in the digital implementation). So we would have
> another cool filterset, let's say 'statelp', 'statebp', 'statehp',
> 'statebr'. 

What I was thinking of was an output structure like the LPC opcodes,
where you have multiple outputs that are available, i.e.:

alow, aband, ahigh   statevar   ain, kfreq, kq, [imode]

The notch mode would come from mixing the highpass and lowpass outputs,
instead of having a preset notch output. A nice feature of this is that
by varying the ratio of highpass to lowpass, you can sweep the notch
frequency up and down, like a 2-pole phase shifter (this feature was
found on the Oberheim SEM module).

Since I think a state-variable filter is not as prone to overflow as a
reson filter, I think that imode could be an optional argument, used to
select a nonlinearity in the feedback loop. This is apparently useful
for generating chaotic waveforms, and might be a nice "chaos processor"
for live and/or sampled signals.

I am going to dig through my old Electronotes issues - I know that there
was a 4-pole state-variable circuit in there, that might be translatable
into digital. This would be very cool, as the output curves of the
circuit were very interesting.
  
> > Does the moogvcf opcode self-oscillate? I am under the impression that
> > no digital filter self-oscillates without an initial source of
> > excitation, but I could be wrong.
> 
> It does self-oscillate, yes. I think that the moogvcf is not a classical
> digital filter, it's a simulation of an analog circuit filter

I wonder, though, if it truly self-oscillates - in other words, can you
use it with no input, as a sine wave source, like a real Moog filter? I
guess I could try it out.