AW: PWM and sinusoids

Haible Juergen Juergen.Haible at
Wed Mar 25 12:12:38 CET 1998

	>ust 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
	>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
	>almost as cool as modulating the VCF at audio frequencies. 

As cool ? Much better, if you ask me ...
Commercial synths: Prophet 5. (Can't await to get mine back next

	>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
	>I've always used triangle, since its easy to build a
integrator/schmitt LFO.

I was surprised, too, when I got my CS-50. Sine waves make
a big difference when used for PWM.

Some other interesting PWM variations:
(1) If you have separate manual PW and PWM knobs, you can easily
get above 100% or below 0%. Then the sound is suddenly cut off.
This can be useful, especially when you use an envelope to modulate the
pulse width: you can produce a delay effect for the second oscillator.
The CS-50 takes another path here: the combined (manual + modulation)
control voltage is limited before it goes to the VCO. So you can't go
above 90% and below 10%. This has its uses, too: You can make  wild
fader moves without loosing the signal (more important on 1VCO machines),
and the modulation waveform is distorted (i.e. no more sine), when you hit
the limits. Very useful.
(2) If you have the same PWM rate and depth over the whole keyboard
range, it tends to sound quite detuned in the low range. So you'd like
to adjust the LFO frequency with the keyboard CV. But full 1V/Oct tracking
is way too much here. The OB-8 has (1Oct LFO) / (4OctVCO) tracking,
which is a very "musical" compromise, too. OTAH, I often prefer to adjust
PWM-LFO rate manually while playing. One of my favorite techniques for
solo lines: Start with fast LFO PWM and slow the modulation
rate down on sustained notes.


More information about the Synth-diy mailing list