[sdiy] PWM audio distortion explanation

[Richie Burnett]

Eric Brombaugh wrote:

> ...Since many forms of PWM are asymmetrical (ie the rising edge always 
> happens in the
> same place and the falling edge is moved) then they're also
> modulating the phase of the harmonics. And therein lie the
> distortion mechanisms...

That's it precisely!  Phase modulation theoretically produces an infinite 
collection of sidebands that spread out either side of every harmonic of the 
carrier frequency.

If the modulation depth is high, and there's not much space between the 
audio frequency and the PWM carrier frequency, then the sidebands on the low 
side of the carrier can easily venture down into the audio band!

-Richie,