Definition
Mid/Side (M/S) processing splits a stereo signal into two components: Mid (what is identical in both channels, the center) and Side (the difference, what is at the sides). Formulas: M = (L + R) ÷ 2, S = (L − R) ÷ 2; to return to stereo: L = M + S, R = M − S. An M/S EQ lets you act separately on the center (vocal, kick, bass, snare) and on the sides (wide guitars, pads, reverbs, cymbals) of an already finished mix.
What is in the Mid and in the Side
- Mid: lead vocal, kick, bass, snare, centered leads, the common part of all instruments.
- Side: panned elements, differences between doubles, stereo reverbs and delays, wideners, overhead cymbals.
Typical uses in mastering
| Move | Channel | Indicative values | Why |
|---|---|---|---|
| HP on the side lows | Side | 80–150 Hz, 12–24 dB/oct | mono lows, more punch, vinyl/club compatibility |
| Air on the sides | Side | shelf +1/+2 dB from 10–12 kHz | openness without hardening the vocal |
| Pad/guitar mud | Side | −1/−2 dB at 250–400 Hz | cleanliness without hollowing out the center |
| Vocal presence | Mid | +0.5/+1.5 dB at 2–4 kHz | vocal more forward |
| Sibilance | Mid | de-esser or dynamic EQ 5–8 kHz | the vocal is in the center |
| Cymbal harshness | Side | dynamic EQ 6–10 kHz | wide cymbals less aggressive |
| Weight | Mid | +0.5/+1 dB at 50–80 Hz | more present center lows |
Precautions
- Small moves: ±0.5–2 dB; M/S noticeably changes the stereo image and the balance.
- Phase and artifacts: with minimum-phase EQs, steep cuts in M/S can create small alterations of the image; linear-phase EQs avoid relative shifts but introduce latency and pre-ringing.
- Width: raising the Side makes the mix wider but weakens it in mono; check the correlation meter and mono.
- Don't repair the mix: if the vocal is 3 dB too low, that's a mix problem.
- Listen to the channels in solo (many EQs let you hear only M or only S) to understand what is really there.
Tools
FabFilter Pro-Q 3/4 (each band in Stereo, Mid, Side, Left, Right mode), iZotope Ozone (M/S EQ and dynamics), Brainworx bx_digital, Maag EQ4 M/S, Kirchhoff-EQ, TDR Nova (free, dynamic EQ with M/S). In FL Studio, Parametric EQ 2 has no per-band M/S mode: you can build an M/S setup in Patcher with Fruity Stereo Shaper (M/S encoding), two separate EQs, and the decoding, or use external plugins.
Example in A minor
Master of a pop-dance track at 122 BPM with wide guitars and a centered vocal: Side HP 110 Hz 12 dB/oct (the guitars and reverb had lows at 80–100 Hz), Side shelf +1.5 dB from 11 kHz, Mid +0.8 dB at 3 kHz with Q 0.8, Mid dynamic EQ −2 dB at 6.5 kHz on the sibilants. The result: a firmer center, more open sides, tighter lows; in mono the track loses only 0.5 dB of loudness.
Common mistakes
- Using M/S as a universal widener: wide masters that are empty in the center.
- Side cuts that are too high (above 200–300 Hz): the mix narrows and loses warmth.
- Not checking in mono and on different systems.
Exercise
- Listen to your master in Mid only and in Side only for a minute each: describe what is there.
- Apply only the HP on the Side at 100 Hz and compare at equal loudness (A/B): how do punch and clarity change?
Links
“Mono Compatibility”, “Stereo, Panning, and Image”, “Mastering Chain and Translation”, “Mastering for Vinyl”, “Mastering for Clubs”.