New Filter Designs

[jhaible]

That's one of the things I always wanted to built myself.
And one that I do not fully understand either.

If you clock it at ultrasonic rate, is it supposed to be
a BP filter or a comb filter ?

I can imagine some foldover taking place when clocked
at audio rate which *might* transform a BP response
into a multi-BP (or comb filter) response. In any sampled
system, you have a frequency response that is periodic -
but you will only notice that if your input signal goes above
the nyquist limit. This is not the same as aliasing - but you'd
have some of this as well, I'm sure. BTW, how's the clock
feedthru when you clock it at audio rate ?

JH.




----- Original Message -----
From: Grant Richter <grichter at execpc.com>
To: <synth-diy at mailhost.bpa.nl>
Sent: Monday, August 16, 1999 9:07 PM
Subject: New Filter Designs


> 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.
>
>