[sdiy] PWM audio distortion explanation

[rburnett at richieburnett.co.uk]

Stumbled upon this good explanation of why simple PWM inherently 
distorts audio reproduction:

http://www.openmusiclabs.com/learning/digital/pwm-dac/pwm-distortion-analysis/

Might make interesting reading for anyone who's used a PIC or Arduino to 
generate audio waveforms using PWM, and been left scratching their head 
for an explanation for unexpected distortion products.

-Richie,