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In signal processing, downsampling (or "subsampling") is the process of reducing the sampling rate of a signal.

Downsampling is usually done to reduce the data rate or the size of the data. The downsampling factor (commonly denoted by M) is usually an integer or a rational fraction greater than unity. This factor multiplies the sampling time or, equivalently, divides the sampling rate. For example, if compact disc audio at 44,100 Hz is downsampled to 22,050 Hz before broadcasting over FM radio, the bit rate is reduced in half, from 1,411,200 bit/s to 705,600 bit/s, assuming that each sample retains its size of 16 bits. The audio was therefore downsampled by a factor of 2.

Source: Downsampling.