inverse multi-mode filters?!?!

[Don Tillman]

Since we're talking about multi-mode filters, I thought I'd go off on
a tangent...  I came up with a cool idea for a synth module the other
day, and I thought you folks would get a kick out of it.

Y'know how you can build a state variable filter to generate
simultaneous Low Pass, Band Pass, and High Pass outputs?  And clever
folks will even combine those to get Notch and All Pass (phase
shifter) outputs?

My idea is a voltage controlled filter module with only one output,
but with *inputs* for Low Pass, Band Pass, and High Pass modes.  So
the stuff below resonance would come from the Low Pass input, around
resonance would come from the Band Pass input, and above resonance
would come from the High Pass input.  And it would be an easy matter
to pipe a signal into more than one for Notch and All Pass modes.

(One "practical" application that comes to mind immediately is to use
this as an interesting way to mix guitar pickups.)

Electrically it's not too difficult.  You could use a standard biquad
circuit for two of the modes and handle the rest algebraically.

Then I thought it would be great to have both multimode inputs and
outputs, but as far as I can tell that's just not possible.  And I'm
not sure it makes much sense either.

  -- Don