[sdiy] SAW core VCO flyback time

[Donald Tillman]

> On Aug 30, 2016, at 11:33 PM, Andrew Simper <andy at cytomic.com> wrote:
> 
> Totally agree here Don, but one thing left to talk about is PWM. I
> have noticed that on a lot of Tri cores the Sqr is derived from the
> Tri, not the Saw, is there any reason for this? If you use the Tri you
> can't get that lovely PWM vibrato sound. In fact perhaps even have a
> switch so you can pick between Saw or Tri for the source of the Sqr.

For an audio VCO, a PWM derived from a triangle wave is "correct" as the fundamental of the PWM waveform is tightly phase aligned with the fundamental of the triangle.

A PWM derived from a sawtooth has phase of the fundamental and harmonics moving around with the control voltage.  It may sound nice, but it's as undocumented side effects go, it's pretty crazy.  (Which would suggest a switch for the feature.)

For an LFO, we don't care about the fundamental and harmonics, so in that case the PWM is derived from the sawtooth so both waveforms start their action at the same time.

  -- Don

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