Matrix MS to Stereo: Quick Guide to Decoding Mid-Side RecordingsMid–Side (M/S) recording and processing is a powerful technique for capturing and shaping stereo imagery. It separates a stereo signal into two components — the Mid (M) channel, which contains the information common to left and right, and the Side (S) channel, which contains the difference between left and right. A Matrix MS to Stereo workflow decodes those components back into a conventional left/right stereo signal so you can listen, mix, and master in familiar stereo terms. This guide explains the theory, practical decoding methods, plugin and DAW approaches, troubleshooting tips, and creative uses.
Why use M/S at all?
- Mono-compatibility: The Mid channel preserves the elements that will remain when summed to mono, while the Side channel contains stereo-specific information. This helps maintain clarity when tracks are collapsed to mono.
- Precise stereo control: You can adjust the stereo width by changing the level of the Side relative to the Mid without altering phase relationships.
- Targeted processing: Apply EQ, compression, or saturation independently to the Mid or Side to shape center and peripheral elements separately.
- Microphone flexibility: M/S microphone pairs (figure-8 for side and cardioid for mid) are phase-coherent and offer accurate stereo imaging that’s easy to adjust post-recording.
The theory: how Matrix MS encoding/decoding works
Encoding (to M/S)
- Given a conventional stereo pair L and R, the Mid and Side channels are computed as:
- M = (L + R) / 2 (the sum, representing common content)
- S = (L – R) / 2 (the difference, representing stereo information)
Decoding (to L/R)
- To restore Left and Right from Mid and Side:
- L = M + S
- R = M − S
These equations assume the encoder used a 0.5 scale on sum/difference. Some hardware or plugins omit the ⁄2 factor; always check gain structure. If the encoder used M = L + R and S = L − R (no ⁄2), then decoding must use L = (M + S) / 2 and R = (M − S) / 2, or adjust gain stages accordingly.
Practical decoding methods
1) Dedicated M/S plugins
Most M/S-capable plugins (EQs, compressors, and M/S encoders/decoders) include a decode function to convert M/S back to L/R. Workflow:
- Insert the M/S plugin on the stereo track.
- Set it to “Decode” or switch to stereo output mode.
- Verify levels with a stereo meter and listen in mono to ensure no large cancellations.
Pros: Quick, often includes extra controls (width, balance, solo M/S). Cons: Plugin-specific behavior; watch gain staging.
2) Manual routing in a DAW (no plugin)
If your DAW allows bus routing and polarity inversion, you can decode manually:
- Create two mono tracks: Mid and Side (or use two aux sends).
- Route Mid to both Left and Right outputs normally.
- Route Side to Left and Right but invert phase on the Right channel.
- Sum Mid + Side to Left output and Mid − Side to Right output by adjusting send levels so they match the encoder’s scaling.
- Use gain trim to match levels and prevent clipping.
This mirrors the math: L = M + S; R = M − S.
3) Matrix technique using simple stereo routing (quick trick)
- Place Mid on the stereo bus center (both channels equally).
- Place Side on stereo bus but pan Side hard left for the left output and hard right inverted for the right output, or use phase invert on one side.
- Adjust levels/scale as needed.
Step-by-step example in a common DAW (conceptual)
- Import or locate your M and S tracks (either recorded as separate files or produced by an encoder).
- Create a stereo bus or master track for decoded output.
- Route Mid track to the bus stereo outputs normally.
- Route Side track twice or to two mono auxes:
- For the Left aux: send Side with polarity normal.
- For the Right aux: send Side with polarity inverted.
- Set send levels so Mid and Side are balanced (if the encoder used 0.5 scaling, boost or cut to compensate).
- Listen in stereo and mono; adjust Side level to control width.
- If necessary, apply M/S-specific EQ or compression before decoding for targeted processing.
Common problems and troubleshooting
- Loudness mismatch: If decoded audio is too hot or quiet, check whether the encoder used the ⁄2 scaling. Apply a trim of +6 dB or −6 dB as needed.
- Phase cancellation/holes when summed to mono: Usually caused by incorrect polarity or mismatched scaling. Verify the Side channel polarity on one of the decoded channels.
- Imaging off-center: Confirm Mid channel is routed equally to both L and R and Side is applied with opposite polarity to R (or vice versa).
- Clipping after decoding: Reduce gain on Mid/Side or apply a limiter; consider decoding at headroom and then normalizing.
Plugins and tools to consider
- Dedicated M/S tools: Most major plugin suites (FabFilter Pro-Q, Brainworx bx_digital, iZotope Ozone) include M/S features.
- Free options: Some free utilities and small M/S encoder/decoder plugins are available; they work well for basic decoding and educational purposes.
- DAW-native: Reaper, Pro Tools, Cubase, Logic Pro — all support M/S processing either natively or via routing tricks.
Tips for mixing and mastering with M/S
- Use mild S-level boosts for perceived width; large boosts can make elements lose focus or introduce phase issues in mono.
- Apply surgical EQ to the Side channel to remove conflicting frequencies (low rumble or harsh high-mid sibilance) without touching the center.
- Compress Mid and Side differently: gentle glue on Mid, slower or more subtle compression on Side to preserve transients.
- When mastering, use M/S carefully: small adjustments can produce large spatial effects.
Creative uses
- Stereo widening: raise Side level or use stereo-specific saturation on Side to enhance ambience.
- Vocal focus: reduce Side energy in the vocal range to keep a vocal centered and clean.
- Reverb shaping: send only Side to certain reverbs or delay taps for a wider perceived space while keeping the dry Mid tight.
Quick checklist for decoding M/S to stereo
- Confirm whether encoder used 0.5 scaling and adjust gains accordingly.
- Route Mid equally to L and R.
- Route Side so it adds to L and subtracts from R (phase invert one side).
- Check mono compatibility.
- Adjust Side level to control stereo width.
- Apply targeted processing before or after decoding as needed.
Decoding M/S to stereo is a straightforward process when you keep the math in mind: L = M + S and R = M − S (or adjust for any scaling the encoder applied). With proper gain staging, routing, and sanity checks (mono, phase, levels), M/S gives you powerful, surgical control over stereo image and mix balance.