Mid/Side Signal Processing

During the mastering process, an imaging plug-in on the stereo main output bus may be used to widen (and sometimes narrow) the stereo sound.  This was described in the last post, but the technical aspect of how this imaging plug-in works was deferred to this post.  Mid/Side signal processing of the stereo image is a clever technique that is used for microphone recordings, mixing tracks, and mastering of the main output bus.  To get a sense of the basic concept behind Mid/Side processing, I've used some simple algebra to track the signal voltage amplitudes of the left and right stereo channels through the imaging plug-in:

The imaging plug-in on the stereo main output bus works to widen (and sometimes narrow) the stereo field in a number of different frequency bands by applying different amounts of gain to the side channel signal in each of these frequency bands.  In a way, this is like having a broad graphic equalizer (EQ) in the side channel signal processing chain.  Keeping the bass fairly focused near the center may actually require applying negative gain (in dB) in the bass frequency band to narrow its stereo image.  Applying positive gain (in dB) in the higher frequency bands will widen the top-end and give the illusion of a wide mix.  

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