AW: Weird voltage controlled Digisound VCF idea

[Haible Juergen]

	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?
>
>	Off the top of my head, this would need each pole of the switch 
replacing
>with a CMOS switch array (4016, 4066) with one side of all the 
switches
>commoned. To drive the switches, rig up an AD converter (say, 8bit) 
which
>receives the CV input, and then fires the assorted CMOS switches 
which are
>wired to the AD digital outputs. By playing around with the digital 
wiring,
>you could make a steadily increasing control voltage step through 
4pole HP,
>2pole HP, 2pole BP, 4pole BP, 2pole LP, 4pole LP and notch (or 
whatever
>sequence you desire).
>
>Is this useful or bonkers?

Ok, hard switching of filter modes may be interesting, especially when 
you can do the switching
at high rates (switch thru filter modes at audio rate by using another 
VCO etc.)

BUT what you'd *really* want, is a smooth crossfade between filter 
modes.
Works at audio rate as well, but is also nice with slow modulation 
sources, as you won't hear
sudden changes and clicking.
Use one envelope to control the cutoff frequency, and a second 
envelope to control filter modes.
Must have heard it to believe it!

JH.