AW: PWM and sinusoids

Jim Johnson jamos at
Wed Mar 25 14:02:18 CET 1998

>>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.

This makes sense when you consider the math, though. Anything that involves
delay (chorus, flanging, or PWM - in essence, you're changing the "delay"
of one edge of the pulse) is going to have an aural effect much like a
pitch shift that is proportional to the derivative of the modulating
signal. (The Doppler effect is an example of this). The derivative of a
triangle wave is a square wave, which results in audible discontinuities
whenever the triangle changes direction. But the derivative of a sine wave
is a cosine wave, which is just a phase-shifted sine - no discontinuities

This is why the difference between a sine and triangle is more noticeable
on PWM that on pitch or filter modulation.

I knew three years of calculus would come in handy one day :')

Jim Johnson 
Metaphoric Software
Makers of Techno Toys
Software for Electronic Music
info at

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