Supersonic frequency source

I’ve been reading/watching a lot about resolution and DAC design. Something that I am not understanding is the origin of the supersonic frequencies that a DAC has to low pass (the 16/44.1 brick wall filter at ~ 22K for example). Are these higher than ~20K frequencies something that was originally picked up by the mic? Is it unavoidable noise that the recording/encoding/decoding electronics themselves generate? Is it just basic background noise that exists in the universe by default?

It an artifact of changing sample rates or going from digital to analog. Sampling can only represent frequencies up to one half of the sample rate. But those same samples also represent other frequencies at the original frequency plus all multiples of the sample rate, here are just two:


All of the ones you don’t want need to be filtered out.

“But those same samples also represent other frequencies at the original frequency plus all multiples of the sample rate”

Ah, gotcha. Thanks.