[sdiy] new CGS filter

[Fahl, Romeo]

I know I've read about this filter before.  I searched through the synth
DIY archive for Bi-N-Tic:
http://www.retrosynth.com/synth-diy/search/search.cgi

and found a post from the venerable Grant Richter:

To: synth-diy at mailhost.bpa.nl,
From: "Grant Richter" grichter at execpc.com,
cc: 
Subject: New Filter Designs 
Date: Mon, 16 Aug 1999 14:07:27 -0500 


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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. 

 

> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl 
> [mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of R. Drake
> Sent: Monday, April 25, 2005 4:00 PM
> To: Synth-Diy
> Subject: [sdiy] new CGS filter
> 
> all,
> 
> happened to note that ken stone has recently added a new 
> filter to his selection of wonderous designs:
> 
> > http://www.cgs.synth.net/modules/cgs57_bintic.html
> >
> > CGS57 -- Bi-N-Tic Filter
> >
> > This is a strange mixture of a VCO and a switched capacitor filter 
> > based on the filter presented by Jan Hall in Electronotes. 
> If you are 
> > after something different, this fits the bill. After all, 
> how many filters have a sync input?
> > 
> > This filter is based on the original idea by Jan Hall, as 
> presented in 
> > Electronotes EN92 P14-15, which was in turn based on an 
> idea in a 1974 
> > issue of Electronic Design on a Biquart (twice the order) filter.
> 
> probably early yet to ask if anyone has built ken's version, 
> but i wonder if any of you esteemed long-timers had any 
> experience with the original jan hall design?  or thoughts on 
> the ken's implementation.  comb filter?
> ringing bursts?  mmmm...
> 
> also noted there are still 4 modules "missing" from the list, 
> assuming sequential numbering... i kind of like the mystery.  
> anyone care to guess what might be in the cooker?
> 
> bbob
> 
> 
>