scream

[Sean Costello]

Sean Costello wrote:
>
> I am going to be coding the state-variable filter into Csound soon (the
> Chamberlin topology, as it is called, due to it's description as a
> useful digital filter structure in Hal Chamberlin's Making Music with
> Microprocessors). It would be trivial to experiment with various
> nonlinearities in the feedback loop. Unfortunately, nonlinearities in
> feedback loops in the digital realm aren't nearly as nice as feedback
> nonlinearities in the analog realm - in the digital world, you get all
> sorts of aliasing, clipping, amplitude growing without bound, etc.
> Still, I'll give it a try, and report the results.

Well, the state-variable filter is coded, and sounds pretty good -
although it is far too clean. For some reason, the Moog topology sounds
much warmer, even in the digital realm. Anyway, tried a few
nonlinearities in the feedback loop, in the location where Dan Slater
located the nonlinearities in the Ueda Attractor circuit.  Here's the
results so far:

- Using x to the 3rd power, the volume went to infinity in 2 samples.
- Using |x|, the volume blew up in about 100 samples.

So far, this seems like a situation where the digital version of a
circuit is unable to duplicate an analog circuit.  I suppose I could
normalize the input to the filter, but this would be difficult for live
signals.  Anyone know of any good bandwidth limited, amplitude limited
nonlinearities that would work in a feedback loop for digital systems?
Sounds easy enough. ;)

Sean Costello

P.S. I know John Fitch has coded a non-linear filter into Csound - I
need to experiment with that, although I am not sure if Fitch's filter
has an analog equivalent.

P.P.S. What do the diodes in the Q feedback loops of some state-variable
filters do? How does it change the sound?