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From Matrixed MS to Stereo: Plugin Picks and Practical Tips

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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.

  1. Import the stereo file (L/R) onto a stereo track.
  2. Duplicate the track so you have two identical stereo tracks (A and B).
  3. On track B, invert the phase of the right channel only.
  4. Pan track A hard left and track B hard right.
  5. 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.
  6. 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

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

  1. Always check mono compatibility first. Decode to M/S and solo the Mid; if essential elements disappear, you likely have matrixing issues to correct.
  2. Use gentle EQ on the Side channel to tame extreme highs or resonant frequencies that cause harshness when widened.
  3. 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.
  4. If the center is weak, slightly increase Mid level (+0.5–2 dB) rather than over-widening Side.
  5. For vintage recordings, mild saturation on Mid can add presence; harmonic excitement on Side can enhance perceived space.
  6. When re-encoding to stereo, perform a phase-correlation and mono-sum check to avoid cancellations.
  7. 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:

  1. Insert MSED (or similar) and decode to M/S.
  2. Solo Mid: apply parametric EQ to add clarity (e.g., +1.5 dB at 3–5 kHz), gentle compression if needed.
  3. Solo Side: HPF at 150 Hz, reduce 3–6 kHz harshness by −1.5 dB, add stereo reverb or width processing lightly.
  4. Blend M and S back, check mono, adjust overall balance, export.

Example B — Creative remix from matrixed stems:

  1. Decode to M/S and export Mid and Side stems as separate files.
  2. Process Mid for rhythm and vocal clarity, process Side for ambience and spatial effects.
  3. 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.

  • 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.

  • 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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