Oversampling is all about processing signals at a higher sample rate than the original signals. This is an important technique that can be used to remove aliasing from non-linear processes such as modulation or distortion.
Oversampling is at least a two-step process. First the sampling rate is increased. Let’s call this process upsampling – or more precise: interpolation. The purpose of upsampling is to increase the sampling rate while preserving the original signal, and only the original signal.
After some signal processing is carried out at the increased sampling rate, the original sampling rate must be restored. This is done by the downsampling process.
As soon as the upsampled signal was processed in some way, decreasing the sampling rate again might not be trivial anymore. If the processing was nonlinear in any way, it will have added additional frequency content above the original Nyquist frequency. In order to satisfy the sampling theorem, the downsampling process must thus make sure that these frequency ranges are filtered out before decreasing the sampling rate. **Excerpts from The Science of Sound (https://science-of-sound.net/2016/07/how-does-oversampling-sound/)
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