New Filter Designs

[Grant Richter]

This discussion has expanded to the "Multiplexed Sample and Hold Resonator"
thread and the "Multipath Filter?" thread.

Sorry I keep changing the thread title but that also
shows refinement of what effect is actually taking
place (or we think is taking place).

Suffice to say the original design from Jan Hall
is being generalized to a new type of filter.
The original design was an experiment that accidentally
stumbled onto a "multibranch" or "multipath" processor
design that is not specific to the state variable
implementation.

----------
> From: jhaible <jhaible at debitel.net>
> To: Grant Richter <grichter at execpc.com>; synth-diy at mailhost.bpa.nl
> Subject: Re: New Filter Designs
> Date: Thursday, August 26, 1999 2:03 PM
> 
> 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.
> >
> >
>