Digital Emulation of Diode Ladder Filters (was Re: [Moog synthesizers]? where!!!

[Sean Costello]

Paul Maddox wrote:

> >Pink Floyd ceased to exist without Syd Barrett, MAN!
> >
> 
> somehow I dont really think this is exactly on topic.

Well, Pink Floyd used Moogs and Synthis, and, uh, Syd Barrett used an
echo which was a Binson Echorec, but he COULD have made it, or maybe
modified it, or...[mumble]

OK, on-topic but digital: Anyone have any advice for digital emulation
of a diode-ladder filter? I know that the poles move around all sorts of
weird ways, but how much of this can actually be HEARD? Can the human
ear really hear the differences in phase and/or slope? Do these changes
cause differences when FMing the filter?

>From what I understand, the migration of the poles means that the filter
will not oscillate when feedback gain (g) equals 4, and that the gain
must be much higher (16 or so?) for self oscillation to occur. What
changes does this make in the sound? My guess is that it would add quite
a bit of distortion to the sound, as the feedback gain would cause some
serious overdriving of the differential pair. What other changes would
occur?

As far as actually implementing the filter, I don't know if you would
really need to emulate the ladder structure to get the sound (i.e. using
4 1-pole filters with an overall feedback loop). The digital realization
of the Moog filter from CCRMA sounds pretty good, but the conversion
from analog to digital uncouples the independence of the frequency and Q
controls. I managed to get a very analog filter sound in Csound a few
days ago by taking several bandlimited pulse waves (which sound like
sawtooth waves, without the aliasing problems), hard limiting them (i.e.
distortion with a very harsh onset), and running the output into a
2-pole resonant lowpass filter that I coded from a design in Hal
Chamberlin's book. The output of this was hard clipped, and run into
another 2-pole lowpass filter with the same characteristics of the
first. The result was a very smooth and intense harmonic sweep, with
none of the characteristic "brittle glass" sound of some other digital
resonant filters.  I think the key is to incorporate clipping into the
structure, with the amount, location and nature of the clipping the key
determinant of the sound.

Back to Pink Floyd: In "Live at Pompeii," the alternate take of "On the
Run" ends with Waters sweeping an oscillator up in pitch, while another
oscillator remains steady. The sound is HUGE, with lots of
intermodulation distortion. I think that this would be the clipping from
the filter, since it is definitely smoother than ring modulation, and
sounds like the "overdrive" of my Moog Rogue. Is this the normal sound
of a diode ladder filter? Or is something else going on in this clip?

Any and all advice will be eagerly received.

Sean Costello