Here we will say about oversampling - sample rate conversion multiplexing. Here we watch how changed spectrum audio for multiplexing sampling frequency.
Usually multiplexing of sample rate released as inserting ZERO SAMPLES (virtual samples) between "real" source samples.
After oversampling spectrum mirrored to upper half of old sample rate fd. So we get mirrored copy of source audio spectrum (upper part of picture).
It is non-linear distorttions in output (oversampled) audio signal.
For getting non-distorted (almost non-distorted, of course) oversampled audio signal, after oversampling need apply low frequency filtration with band from 0 to fd/2.
So we get almost clean oversampled source audio signal (music).
About downsampling read here...
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