Weird voltage controlled Digisound VCF idea

[David Halliday (Volt Computer)]

> >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!
> 
> 
> 
> 
> This could be done by using a very high frequency to control the
> switches and then varying the duty cycle of that freq...  Use a
> triangle wave and two comparitors - your voltage control sets the
> level of the comparitors - you could even use a pot to set a
> "deadband" point at which both signals are being passed...