Weird voltage controlled Digisound VCF idea

[Don Tillman]

   Date: Tue, 29 Apr 1997 13:32:21 +0200
   From: "Matthew S. Padden" <@horus.sara.nl, at pegasus.huddersfield.ac.uk:m.s.padden at huddersfield.ac.uk>

   The recent thread about the Digisound VCDO made me remember an idea I
   had for the Digisound 80-7 VCSVF (the one based around the
   SSM2040). It uses a 2-pole 7-way rotary switch to choose the filter
   response (2pole LP, BP, HP, 4pole LP, BP, HP, and notch). I thought:
   Why not have voltage controlled selection of the filter type?

You've already got that to a certain degree for a 2-pole state
variable filter.  

"Huh?"

Well.... by that I mean that a 2-pole state variable filter really
only has two simultaneous outputs, LP and BP, and all the other modes
are derrived from those.  HP = input - LP - BP, notch = input - BP,
phase shifter = input - 2BP.

So crossfading between some of those is pretty straightforward.

(There's an issue of adjusting the bandpass level for Q; that depends
on the implementation.)

  -- Don