[sdiy] Ridiculously PC (Phase Correct)

[Don Tillman]

   Date: Sun, 04 Nov 2001 01:20:46 +0100 (CET)
   From: Magnus Danielson <cfmd at swipnet.se>

   > For audio signals, the fundamental components of all waveforms are in
   > phase, so that panning, switching, or mixing the individual waveforms does
   > not result in any FM artifacts or unexpected changes in the amplitude
   > of the fundamental.

   You mean Phase Modulation, not Frequency Modulation. If two of the
   output waveforms have different phases for the same base or overtone
   will the change in mix between them cause a shift in the phase of the
   output waveform. 

I meant FM generically (like Chowning does!); PM is certainly more
accurate.

[equations omitted]

   According to this little investigation only the sawtooth is of phase
   wise, if you derive the PWM and sine from the triangle waveform.

Yes, the sawtooth is the problem.

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.

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've never heard of a commerical VCO with the correct saw/triangle
phase relationship.

   Phase-shifting the sawtooth waveform can be done using something
   similar to my sawtooth phaseshifter at:
   http://home.swipnet.se/cfmd/synths/schematics/

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.

  -- Don

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