From Matrixed MS to Stereo: Plugin Picks and Practical TipsMid/Side (M/S) recording and processing is a powerful technique that separates an audio signal into a mono “Mid” component (the center information) and a stereo “Side” component (the difference between left and right). Matrixed M/S — where the M and S channels have been combined (matrixed) into standard left/right signals — is commonly used in vintage recordings, some broadcast workflows, or when an M/S-encoded file has been distributed as L/R. Converting matrixed M/S back to a true stereo pair (or extracting Mid and Side for independent processing) can restore control and unlock creative mixing possibilities.
This article covers:
- How matrixed M/S works and how to recognize it
- Manual decoding vs. plugin-based decoding
- Recommended plugins for decoding and M/S processing
- Practical tips for mixing, mastering, and restoring matrixed material
- Workflow examples and troubleshooting
How matrixed M/S works (brief primer)
A classic M/S encoder creates left and right channels from Mid (M) and Side (S) signals using: L = M + S
R = M − S
When you encounter a matrixed M/S file, those L and R channels already contain the encoded M and S information. To recover the original Mid and Side components, you apply the inverse: M = (L + R) / 2
S = (L − R) / 2
Understanding this math helps when you need to perform manual routing in a DAW or when diagnosing phase or imaging issues.
How to tell if audio is matrixed M/S
Signs that a stereo file is matrixed M/S:
- Unusual stereo width that collapses or widens dramatically when summed to mono.
- Center content (vocals, kick, snare) is oddly quiet or overly wide compared to expectations.
- Phase meter shows large anti-phase content between channels.
- You know the source: broadcast archives, certain radio recordings, and some hardware recorders use matrixing.
A quick test: invert the phase of one channel and listen. If the signal largely cancels or changes character drastically, it’s likely M/S-encoded material.
Manual decoding in a DAW (step-by-step)
If you prefer not to use dedicated plugins, you can decode matrixed M/S manually with basic DAW routing and simple gain/phase tools.
- Import the stereo file (L/R) onto a stereo track.
- Duplicate the track so you have two identical stereo tracks (A and B).
- On track B, invert the phase of the right channel only.
- Pan track A hard left and track B hard right.
- Adjust levels: to get true M and S, set both tracks to −6 dB (because M = (L+R)/2 and S = (L−R)/2). Some DAWs let you apply a gain plugin of −6 dB or set clip gain accordingly.
- Now the summed signal of the two mono outputs gives you a representation of M (sum) and S (difference). Route them to separate buses for independent processing, then re-encode or sum back to stereo if needed.
This method is flexible but requires careful gain and phase handling.
Plugin-based decoding: why use plugins?
Plugins simplify routing, provide meters for Mid/Side content, and often include extra tools (EQ, width, saturation) designed specifically for M/S work. They reduce human error and speed workflow.
Key features to look for:
- Stereo-to-M/S conversion and back
- Mid and Side metering and soloing
- Phase correlation meter
- Per-band M/S processing (multiband M/S)
- Transparent (or characterful) processing options
Recommended plugins (free and paid)
Below are solid plugin choices for decoding matrixed M/S and for deeper M/S processing.
Free:
- Voxengo MSED — A widely used, free M/S encoder/decoder with solo/monitor and gain controls. Simple and transparent.
- MeldaProduction MMultiBandMS (free version available) — Multiband M/S with extensive modulation and metering.
- Ozone Imager (iZotope) — Stereo imaging tool that visualizes stereo field and can help identify matrixed content (note: not a pure M/S decoder but useful for imaging adjustments).
Paid:
- Brainworx bx_control V2 — Precise M/S control, excellent monitoring tools, and mid/side soloing.
- FabFilter Pro-Q 3 — Not an M/S encoder per se, but supports Mid/Side processing per band with a clean interface and linear-phase options.
- NUGEN Stereoizer / Halo Upmix — Advanced control for stereo image manipulation and M/S workflows.
- SPL M/S Processor — Hardware-modeled plugin that offers transparent decoding and analog-style treatment.
Practical tips for mixing matrixed M/S material
- Always check mono compatibility first. Decode to M/S and solo the Mid; if essential elements disappear, you likely have matrixing issues to correct.
- Use gentle EQ on the Side channel to tame extreme highs or resonant frequencies that cause harshness when widened.
- Be cautious boosting low frequencies in the Side channel — it can create an unstable low-end and phase issues. Use a high-pass on S around 100–200 Hz if needed.
- If the center is weak, slightly increase Mid level (+0.5–2 dB) rather than over-widening Side.
- For vintage recordings, mild saturation on Mid can add presence; harmonic excitement on Side can enhance perceived space.
- When re-encoding to stereo, perform a phase-correlation and mono-sum check to avoid cancellations.
- Use automation on Mid/Side balance for sections where the stereo image should change (chorus vs verse, solo vs ensemble).
Mastering considerations
- Multiband M/S processing can rescue a thin mono mix by narrowing low-mid S while widening high frequencies.
- Avoid heavy limiting on Side material at mastering — it can squash the stereo image and create pumping artifacts.
- Use a correlation meter to ensure the final track remains safe for mono playback, especially for vinyl or broadcast.
Workflow examples
Example A — Restore a matrixed vintage stereo file:
- Insert MSED (or similar) and decode to M/S.
- Solo Mid: apply parametric EQ to add clarity (e.g., +1.5 dB at 3–5 kHz), gentle compression if needed.
- Solo Side: HPF at 150 Hz, reduce 3–6 kHz harshness by −1.5 dB, add stereo reverb or width processing lightly.
- Blend M and S back, check mono, adjust overall balance, export.
Example B — Creative remix from matrixed stems:
- Decode to M/S and export Mid and Side stems as separate files.
- Process Mid for rhythm and vocal clarity, process Side for ambience and spatial effects.
- Reconstruct stereo with M/S encoder, automate width for dramatic impact in drops or breakdowns.
Troubleshooting common problems
-
Problem: After decoding, vocals sound phasey or hollow. Fix: Check that you inverted the correct channel during manual decode; ensure tracks are at −6 dB if using summing math. Use a correlation meter to diagnose. Apply slight EQ to Mid to restore presence.
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Problem: Low end disappears or becomes unstable. Fix: Apply a high-pass to the Side channel around 80–200 Hz. Ensure Mid carries the mono low-frequency content.
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Problem: Too wide / washed-out mix after re-encoding. Fix: Reduce Side level, tighten Side EQ, or use multiband M/S to narrow problematic bands.
Quick reference: math recap
- Encoding: L = M + S, R = M − S
- Decoding: M = (L + R) / 2, S = (L − R) / 2
Converting matrixed M/S into usable stereo gives you control over spatial balance, corrective EQ, and creative effects. Whether you choose a manual routing approach or a polished plugin workflow, the key steps remain the same: correctly decode, process Mid and Side thoughtfully, and verify mono compatibility before final export.
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