tri 2 saw modualtion

[Don Tillman]

   From: Martin Czech <martin.czech at itt-sc.de>
   Date: Wed, 22 Oct 1997 15:13:02 +0200 (MET DST)

   A while ago I asked, how voltage controlled symmetry  could be achieved
   with a triangle type vco (lfo) when the frequency should stay
   constant.  

[...idea presented...]

   So I don't expect resonable behaviour in audio applications, where
   frequency stability is a must. Symmetry modulation in this case is not
   very interesting, it sounds like a more or less lowpass filtered saw.

Huh?  Really?  Have you tried it?  Doesn't it have some of those
wonderful swooping Bessel characteristics?  How about audio frequency
modulation of symmetry?  I haven't tried it myself, but I can't help
but believe it would be useful.

Here's another aproach:  Start with a sawtooth VCO and build a circuit
to warp the symmetry of that.  

Say you have a 0.0 -> 1.0 volt sawtooth.  Build two VCAs with gains
that go from 0.5 to about 100 or so, one VCA gets that sawtooth and
the other VCA gets the inverse (1.0 -> 0.0 volts).  Add a circuit that
produces the minimum of the two VCA output voltages -- that's the
output.  Then adjust the two VCA gains so that the negative-going VCA
crosses 1.0 volt at the same time the positive-going VCA crosses 1.0
volt.

Creating control volages for those VCAs isn't too difficult:

Say you have a 0.0 -> 1.0 volt symmetry control voltage.  Build two
more VCAs, one gets that control voltage and the other gets the
inverse (1.0 -> 0.0 volts).  Two little opamp feedback circuits
compare their outputs to 1.0 volt and adjust their control voltages so
they are.  Send those control voltages to the VCAs above.

Obviously the VCAs need to be matched.

This circuit could also do some mighty gruesome things to non-sawtooth
waves.

  -- Don