"Fleetie" wrote in message
...

In the real world, the output from the D-A converter running at 30kHz
(remember Nyquist) will not be a 15kHz sinusoid, but will be a 15kHz
square wave.

Absolutely and totally wrong. The output of any proper DA is a smooth wave
with no square waves related to the sampling frequency.

However this does NOT mean you're getting some higher-
frequency information "for free". The output ought to be filtered to
remove ALL components above 15kHz, and when that is done, you will
find that you ARE left with a 15kHz sinusoid again.

A DAC that lacks such filtering is not a proper DAC, which is why its wrong
to say that a DAC has a square wave output.