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Andreas
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Sorry, Bob, that I need to correct you (dealt too many years in synth design and all the Nyquist stuff).
Sure, frequencies above 16-20kHz can’t be heard, depending on age and music volume preferences… But the math behind sampling-therory does not care about this and also will handle frequencies above that margin.
If you do a quality recording of some (natural) instrument at 48kHz, the sample stream likely contains frequencies up to 24kHz (half of sampling frequency). Downsampling to 44.1kHz without previous brickwall filtering at 22kHz will fold back any higher frequency parts back into the allowed range. An unfiltered 23kHz signal will then not only occur at 21kHz but also at 1kHz, which will be audible by hopefully anyone of us.
Even if the energies from these bands are normally very low, they’re easily recognized as wrong, since they’re strongly non-harmonics compared to the remaining stuff.
The sampling theorem isn’t very tolerant in that respect, I’ll tell you… 😉