Mid/Side

Definition

The Mid/Side (M/S) representation describes a stereo signal with a sum channel and a difference channel:

M = (L + R) / 2 S = (L − R) / 2

and we go back to left and right with L = M + S, R = M − S. A variant preserves energy with the factor 1/√2 in place of 1/2. The Mid channel contains what is the same in both channels (centered sources and the common component of side sources); the Side channel contains the differences (side components, ambience, content in antiphase). With 1/2 normalization, P_L + P_R = 2 · (P_M + P_S).

For a source panned with gains g_L and g_R: M = (g_L + g_R)/2 · x, S = (g_L − g_R)/2 · x. A center source has S = 0; a source entirely on the left has M = S. Multiplying S by a factor w controls the width: w = 0 mono, w = 1 unchanged, w > 1 wider. The Side/Mid ratio in dB (10 · log10(P_S / P_M)) measures the width; for channels of equal power it equals (1 − ρ)/(1 + ρ), with ρ the correlation. The same encoding underlies the M/S recording technique (front-facing cardioid or omnidirectional microphone plus a side-facing figure-of-eight microphone).

Parameters and units

• Side / Mid gain: dB.
• Width: % (0% mono, 100% unchanged, 200% Side +6 dB).
• M/S EQ: independent bands on the Mid or Side channel.
• High-pass on the Side: 80–150 Hz, 12–24 dB/octave.
• Measured Side/Mid: about −20 dB or less is nearly mono; −10/−6 dB is a wide image; close to 0 dB is very wide, with a risk of phase problems.

Musical function

M/S processing makes it possible to work separately on the center and the sides: in mastering you center the bass by filtering the Side, add air on the sides without making the center vocal sibilant, reduce mud in the center without emptying guitars and pads, and compress or de-ess only the Mid. In the mix it is used to widen or narrow backing vocal, pad and overhead buses and to control reverbs that are too wide.

Practical example

• Calculation: L = 0.8, R = 0.2 → M = 0.5, S = 0.3; Side/Mid = 20 · log10(0.3/0.5) = −4.4 dB.
• Mastering: high-pass on the Side at 120 Hz, 12 dB/octave; shelving +1.5 dB from 8 kHz on the Side; bell −1 dB at 300 Hz, Q 1, on the Mid. In the analysis the width of the Sub band goes from −14 to −28 dB, that of the Air band from −9 to −7.5 dB; the correlation stays above +0.5.
• Backing vocal bus: Side +2 dB (width 126%) and high-pass on the Side at 200 Hz: the backing vocals widen without adding side energy in the low mids.

Common mistakes

• Raising the Side beyond +3 dB on the whole mix: mono loses level and phase artifacts appear.
• Filtering the Side too high (beyond 250–300 Hz): the image narrows in the mids and the mix sounds closed.
• Using minimum-phase EQs with strong, different moves on M and S: the phase shift between the two channels alters the image; for marked corrections linear phase is preferable.
• Confusing L/R balance with the Side/Mid ratio.
• Treating the Side as a channel independent of the Mid: every change also alters L and R.

How to check it

• App analysis: Side / Mid (total energy) for the overall width; the width of each band (per-band Side/Mid ratio) for the frequency profile; Stereo correlation and Mono sum drop to verify that the intervention does not compromise mono.
• Separate listening: listen to the Mid alone and the Side alone with an M/S plugin (Ozone Imager, FabFilter Pro-Q in Mid/Side mode): the Side should contain ambience and side instruments, not vocal or bass.
• Before and after: analyze the file in the app before and after M/S processing and compare the per-band widths.