Hans Mikelson wrote:
> 
> >I wonder, does anyone know anything about the so-called "warm" or "analog"
> >digital filters:
> 
> The best one in Csound now is probably moogvcf however when compared 1:1
> with a real analog filter the analog sounds:
> 
> More vocal, more "up front", less muddy/mushy.  Not sure why but perhaps
> phase plays a role?

Probably has to do more with the nonlinearities in the circuit
(distortion from the differential pair, distortion between the stages of
the transistor ladder). These would be a bear to implement, because the
distortion from the differential pair, besides being a form of soft
limiting (hard enough to implement digitally), would form a part of the
feedback path in the circuit. Having nonlinearities in a feedback path
is a nice way to get aliasing, amplitude overflow, and other nasty
digital artifacts.  People like Harvey Thornburg at CCRMA are working on
developing bandlimited nonlinearities for just this very reason.

Anyway, the moogvcf opcode still sounds pretty good. Not really like a
Moog filter - more like the Roland SH-101, which has the same design as
the Moog filter as far as having 4 cascaded one-pole filters with an
overall feedback path, but uses an entirely different analog circuit
based on OTA's as opposed to a transistor ladder. It certainly sounds
more analog than the state-variable circuit I coded the other day, which
is useful, but sounds very brittle.

Probably the best way of emulating an analog synth with the existing
opcodes is by using several bandlimited pulse waveforms (gbuzz), mixed
together, run through some sort of nonlinear clipping function, then run
through the moogvcf. The nonlinear clipping function will simulate the
types of intermodulation distortion that will be generated in a Moog
when the input waveforms overdrive the differential input of the filter.
Synths like the Minimoog and Moog Rogue featured this type of distortion
as an integral part of their sound. Running the output of the moogvcf
through distortion would also be useful, as the VCA used in the Minimoog
also displayed nonlinear behavior.