Waveshaping (was: 30 years later)

[Christopher_List at Sonymusic.Com]

  >   1. The tri->sine converter uses a 3080, with a fixed current into the
  >   control pin. What would happen to the output if the current varied - by
  >   varying the voltage? In other words, instead of connecting the 220K
  >   resistor to +15v, connect to the tri-wave output (or something
  else)...

  OK, so I got a chance to try this last night and got some neat results...

  First off, the 3080 will basically work like a VCA - so this is kind of
  like doing amplitude modulation (AM) of a sine wave with the modulator at
  the exact same frequency. The nice thing is that you can get the effect
  from the VCO with no extra chips!

  The control pin of the sine wave 3080 is connected to +V through a 220K
  resistor. I disconnected this resistor and hooked up a 68K to the control
  pin. Then I tried connecting the other end of the 68K to the various
  OTHER outputs;
  Triangle - No real effect
  Square - Sine wave with a flatter section in the middle. This was kinda
  cool. The strong fundimental of the sine wave with a slight buzzy
  overtone.
  Pulse - Sine wave with a "notch" in the peak of the wave. The width of
  the notch was proportional to the pulse width. This was OK, but not great
  - too much drop in amplitude.
  Saw - This was cool. A straight saw wave created a sine wave with a
  verticle jump in the middle. Again, strong fundimental with a buzzy
  overtone. Where it gets to be fun is when you do PWM on the sawtooth wave
  that's being used to modulate the sine wave - result? -  weird PWM sine
  wave! Cool! A strong fundimental with weird overtones. Obviously taking
  this to the next step by doing audio frequency PWM on the sawtooth wave
  (with another VCO) created a really nice rich waveform - yummy yum.

  This last one is definitely useful - the best thing about it - like I
  said - is that it requires no extra parts. A couple of my VCO's will
  definitely have mysterious unmarked switches appearing on their
  faceplates in the near future! In my opion it's things like this that
  make "modular synths" into "secret weapons".

  The best results come from dramatic AM, so I would recommend something
  like a 33K (not 68K) resistor from the sawtooth output to the control pin
  of the 3080. This will create a very big output wave, so you'll also have
  to drop the 39K feedback resistor in the current->voltage opamp to
  something like 15K (these values are approximate). A little DPDT could
  switch in and out the original resistors...

  Sorry I can't give a better description - I suggest anyone else who's
  built one of these VCO's give this simple mod a try and let us know what
  YOU think.

  - Chris