PWM and sinusoids

[Tony Allgood]

Hello all,

This probably won't be new to owners of huge modulars, but I discovered
something really cool when I was playing around with a couple of oscillators
and a comparator. My intention was to show my students how to make a pulse
width modulator, one of the oscillators was at around 440Hz and the other
was a low frequency, 1Hz or so. I hooked up the output of the comparator to
an amp and a speaker, and they were impressed by the resulting sound. Yeah,
nothing new in that I hear you say, just PWM phasing. But when one of them
turned up the frequency of the LFO to audio frequencies... it was me that
was impressed. I couldn't believe the tonal textures that could be attained
by this arrangement. The two oscillators produced wonderful textures when
slightly offtuned, richer still when an octave apart, and amazing when tuned
at fifth. Why was this never included on commercial synths? This is as
almost as cool as modulating the VCF at audio frequencies. I think I'll get
inside my home made synth this weekend and add a new modulation path to the
PW input.

The second thing that amazed me was the difference in using sine waves to
modulate the PW at low frequencies. The effect was so much more musical.
I've always used triangle, since its easy to build a integrator/schmitt LFO.
But, this weekend I'll be converting all my LFO triangles to sine... four
diodes and a few resistors should do it.

Tony Allgood, Cumbria, UK

e-mail: oakley at enterprise.net

My latest project; a rack mounted VCF module with warmth. See it at:

http://aupe.phys.andrews.edu/diy_archive/schematics/effects/filter.html