The second step of mid-side processing is decoding. This involves the process of turning the encoded “mid” and “sides” signals back into a conventional two-channel, left-right stereo signal.

Typically when using MS processing, some other additional type of processing (equalization, compression, etc.) is applied separately to either the “mid” or “sides” signals. Then, it is the processed versions of the signals which get decoded.

The following equations can be used to perform mid-side decoding:

$L = M + S = \frac{1}{2}(L + R) + \frac{1}{2}(L - R) = \frac{1}{2} \cdot L + \frac{1}{2} \cdot L$

$R = M - S =\frac{1}{2}(L + R) - \frac{1}{2}(L - R) = \frac{1}{2} \cdot R + \frac{1}{2} \cdot R$

A basic application of mid-side processing is stereo widening, to adjust the relative amplitude of the “mid” and “sides” channels.