inverse multi-mode filters?!?!

[gstopp at fibermux.com]

     So, let me get this straight - a composite sound is produced that has 
     the low end from one source, the mid from a second, and the highs from 
     a third?
     
     Reminds me of those pictures of people where you put somebody's head 
     on somebody else's body on another person's legs on a different 
     person's feet....
     
     I can imagine doing this with three spearate state-variable filters 
     and a summer, at least as an initial brute-force approach. You could 
     apply the same CV to each so that they sweep together, and if they 
     have VCQ you can control the Q on all 3 at the same time as well. I 
     don't have a clue off the top of my head how to do it with one biquad 
     plus summing, so if this is possible it would be a fun thing to try.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: inverse multi-mode filters?!?!
Author:  don at till.com at ccrelayout
Date:    5/14/96 5:38 PM


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