There seems to be missing one very important point in this discussion of the AD/DA process.

The final step in the digital to analog conversion process always includes a low pass filter between the Nyquist frequency and the word clock frequency i.e. for a 44.1K system, between ~20Khz and 44.1Khz.

This does two very imprtant things:

It strips off the word clock (44.1K etc).

It removes ALL harmonics of the audio above the Nyquist frequency.

Thus - a 20Khz tone stripped of all of it's harmonics equals a pure SINE WAVE.

The raw output of the D/A converter before the low pass, for a frequency approaching the Nyquist frequency (20Khz) is an approximation of a square wave, not a triangle wave, due the sample and hold stage.

[This message has been edited by soundhound (edited November 15, 2002).]