[sdiy] Re: Variable Waveshape LFO

[Don Tillman]

   > Date: Sat, 13 Jan 2007 13:47:57 +0100
   > From: =?iso-8859-1?Q?Fran=E7ois_CHAUVET?= <fchauvet at cegetel.net>
   > 
   > Don Tillman wrote:
   > 
   >  > The two-diodes-and-a pot in the VCO core approach works well,
   >  > but that isn't voltage controlled and may have a slight
   >  > detuning effect on the pitch. So that's good for LFO's with
   >  > manual control.
   > 
   >  > You could do a voltage controlled version of that, with OTA's
   >  > replacing the pot, but there would be a much greater effect on
   >  > the pitch. So that's okay for LFO's with voltage controlled
   >  > waveshape.
   > 
   > A "slight" detuning effect? I did that about 20 years ago, and
   > detuning was well over one octave! Be it with two diodes & a pot,
   > or with OTAs...

That's not right; there are a bunch of commercial products that use
the technique very successfully.  For a one octave detuning it's more
likely you had a design or wiring error.

   >  > My approach is a post-processor on a sawtooth wave, which is a
   >  > little more work, but it's voltage controlled and is
   >  > guaranteed not to affect the pitch. So that's good for audio
   >  > VCO's with voltage controlled waveshape.
   > 
   > This is exactly the technique I use. Pitch is not affected at
   > all, but I still have an enormous glitch at the reset of the
   > (reference) sawtooth wave.

The details of the circuit design need to supress the glitch.  I can't
comment on what your design is doing because I couldn't tell anything
from those images you sent out before.

  -- Don

-- 
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com