New Filter Designs

[Grant Richter]

I have been experimenting lately with new filter designs
based around the state variable filter.

One design that has always intrigued me is from Jan Hall
in EN #95 the "Bi-N-Tic" filter. This is a state variable using
commutating capacitors for the integrators.

To elaborate, build your usual state variable, for the capacitors
in the integrator, use a CD4051 with eight capacitors of the
same value. Power the CMOS off of +/-7.5 volts and multiplex
the 4051s with a counter like a CD4024. You'll need two
4051s and 16 caps. Since the CMOS is powered from +/-7.5
the switch point is at ground, making it easy to clock.

The circuit produces the sharpest comb filters that I have heard.
The sound is very similar to a flanger or phase shifter.
While the original design used an ultrasonic clock,
by far the most interesting sounds are produced by
clocking it in the audio range. It sounds really good
with a rhythm box driving a sequencer controlling the
clock frequency.

Can anybody explain why the commutating capacitors
create comb filters at what seem to be octave intervals
of the clock frequency? My brain doesn't seem to be
able to grok this.