[sdiy] Ridiculously PC (Phase Correct)

[Don Tillman]

   Date: Sun, 04 Nov 2001 14:01:16 +0100 (CET)
   From: Magnus Danielson <cfmd at swipnet.se>

   Don:
   > Historically many VCOs have derrived the PWM square from the sawtooth
   > (MiniMoog, Moog901, Moog921, ARP2600, etc.) creating a waveform whose
   > fundamental component shifts with the pulse width.  It's just as easy
   > to derrive it from the triangle assuring constant phase.

   Indeed. This is also why I chose the triangle as a source in this
   case, since I *knew* that it also gets phasemodulated with the CV and
   not only pulsewidth modulated. 

Yes.  Interestingly, a PWM derrived from a triangle is the right thing
for an audio oscillator, but a PWM derrived from a falling sawtooth
has a constant leading edge, which is what you want for an LFO.

   > The weirdest variation I know of is the ARP2600 VCO with the sine wave
   > polarity opposite the triangle (!!!).  Presumably this make it easier
   > to create an anti-triangle, but it's *so* unintuitive.

   I can't see how it would make it easier to make an anti-triangle, but
   never the less a curious detail.

Just that you can easily mix the triangle and sine together and
effectively subtract the fundamental from the triangle.  No biggie.

   > Yes, though it doesn't seem optimal to have a sawtooth core, and it's
   > attendant reset time issues, when the core sawtooth wave gets chopped
   > up in the phase shifting process.

   Indeed. I find the triangle cores quite interesting in this
   respect. 

Yes...  Hmm, I believe it takes only slightly more effort to derrive a
correctly phased sawtooth from a triangle than it does to derrive an
incorrectly phased sawtooth from a triangle.

And if you're going to have a glitch in a waveform, it's better to
have the glitch in a sawtooth (derrived from a triangle) with its
higher harmonic content than in a triangle (derrived from a sawtooth)
with its lower harmonic content.

So while a sawtooth is the natural choice for a core waveform, the
triangle is starting to look pretty good in this situation.

  -- Don

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