Mid/Side (M/S) is a way of handling a stereo signal by splitting it into a Mid component and a Side component, the latter being the difference between left and right. Mid is what the two channels share; Side is the width that arises from their difference. It lets you adjust the center and the edges separately.
What Mid and Side represent
Mid is the content common to L and R, and it holds most of what sits in the center, such as vocals, kick, and bass. Side is the difference between L and R, containing stereo width, stereo reverb, the variation between doubled takes, and so on. Splitting into M/S lets you adjust by role in ways that ordinary L/R processing makes hard to see.
Relationship to mono
When stereo is summed to mono, the Mid component essentially remains while the Side component weakens. So if you put important low end or the main melody only in the Side, the core of the song can shrink on mono playback. After M/S processing, always listen in mono too and confirm that the central elements hold up.
Using it with EQ and compression
Typical uses include tidying the low end of the Mid only, gently taming the highs of the Side, and widening the edges while keeping the center vocal intact. Large moves, though, can make the stereo image unnatural or emphasize phase problems. Confirm that L/R processing is not enough before reaching for it, and use it only as far as you need.
Care when widening the stereo image
Raising the Side sounds wider in stereo, but it also increases what disappears in mono. Widening the low end too much can produce muddiness or instability depending on the playback system. Keeping the elements the song depends on in the Mid and building width from decorative material makes these decisions easier.
How to check
Compare before and after M/S processing at matched levels, and listen in stereo, in mono, and on both headphones and speakers if needed. If you have a Side-only monitoring option, use it to check that unwanted low end or noise has not crept in. When making a file for others to audition, you can use the audio format converter.
