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audio-mastering-mcp

npm version license

An MCP server that masters audio with a real, professional signal chain — powered entirely by ffmpeg, no DAW required. Give it a rough mix (or a vocal + a beat) and get back a clean, loud, release-ready master, an immersive wide-stereo version, and Dolby-codec exports.

It exists because letting an AI assistant "master a track" usually means hand-waving. This makes it concrete: a fixed, well-reasoned chain (corrective EQ → glue compression → multiband compression → saturation → exciter → stereo stage → limiter), finished with two-pass loudnorm so loudness and true-peak land on an exact target every time.

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Table of contents

• Features
• The mastering chain
• Install
• Use it in an MCP client
• Tools
  • analyze_loudness
  • mix_vocal_over_beat
  • master_audio
  • export_dolby
• Loudness targets
• About "Dolby"
• Requirements
• Troubleshooting
• License

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Features

• analyze_loudness — measure integrated loudness (LUFS), true peak (dBTP), loudness range (LRA), and gating threshold.
• mix_vocal_over_beat — lay a vocal over an instrumental with sidechain ducking (the beat dips under the words), presence EQ and a touch of reverb on the voice. Outputs a 24-bit WAV pre-master.
• master_audio — the full chain with 2-pass loudnorm to an exact LUFS / true-peak target. A spatial option (immersive | subtle | none) controls the stereo image, and it reports the mono-sum drop so you can check mono compatibility.
• export_dolby — encode to Dolby Digital Plus (E-AC-3) or Dolby Digital (AC-3).

Everything is deterministic and parameterized — same input + same settings → same master.

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The mastering chain

master_audio runs this signal flow (industry-standard order, hip-hop-tuned):

input
  │  corrective EQ        high-pass 28 Hz · -2 dB @ 300 Hz (mud) · -1 dB @ 3 kHz (harsh)
  │                       +1.5 dB low shelf @ 90 Hz (warmth) · +2 dB air shelf @ 11 kHz
  │  glue compression     2:1, slow (20 ms / 200 ms), ~1–2 dB GR for cohesion
  │  multiband comp       3 bands (≤150 Hz / 150 Hz–4 kHz / ≥4 kHz) each lightly controlled
  │  saturation           gentle tanh soft-clip for harmonic warmth
  │  harmonic exciter     high-frequency sheen for clarity on small speakers
  │  stereo stage         none | subtle | immersive   (see below)
  │  limiter              driven peak limiter, ceiling -0.9 dBFS
  │  loudnorm (2-pass)    linear normalization to exact LUFS / true-peak target
output  → 320 kbps MP3

Stereo stages

spatial	What it does
none	Leaves the stereo image untouched.
subtle	Light overall widening (extrastereo).
immersive	Keeps the bass mono (solid, centered low end) while widening the highs (stereotools mid/side) and adding a subtle multi-tap air/depth. Biggest, most "premium" feel — at the cost of a few dB of level when summed to mono, which the tool reports back.

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Install

# one-off, no install:
npx -y @ope-olatunji/audio-mastering-mcp

# or globally:
npm install -g @ope-olatunji/audio-mastering-mcp
audio-mastering-mcp

From source:

git clone https://github.com/ope-olatunji/audio-mastering-mcp.git
cd audio-mastering-mcp
npm install && npm run build
node dist/index.js

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Use it in an MCP client

Claude Code (~/.claude.json or project .mcp.json) / Claude Desktop (claude_desktop_config.json):

{
  "mcpServers": {
    "audio-mastering": {
      "command": "npx",
      "args": ["-y", "@ope-olatunji/audio-mastering-mcp"]
    }
  }
}

All file paths passed to the tools must be absolute.

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Tools

analyze_loudness

Measure a file's loudness metrics.

Param	Type	Default	Notes
input	string	—	absolute path to a file

// → 
{ "integratedLufs": -11.7, "truePeakDb": -1.2, "lra": 1.1, "thresholdDb": -21.8 }

mix_vocal_over_beat

Combine a vocal and an instrumental into a pre-master WAV.

Param	Type	Default	Notes
beat	string	—	instrumental path
vocal	string	—	vocal path
output	string	—	output .wav path
vocalDelayMs	number	1200	how long the beat plays before the vocal enters
beatVolume	number	0.30	beat level under the vocal (lower = vocal forward)
vocalVolume	number	1.9	vocal level

The beat is sidechain-ducked under the vocal so the words always cut through.

master_audio

Run the full mastering chain to a target.

Param	Type	Default	Notes
input	string	—	pre-master / mix path
output	string	—	output .mp3 path
targetLufs	number	-10	integrated loudness target
truePeak	number	-1.5	true-peak ceiling (use ~-1.5 so MP3 stays ≤ -1.0)
spatial	"none" | "subtle" | "immersive"	"immersive"	stereo stage
bitrate	string	"320k"	MP3 bitrate

// → 
{
  "output": "/abs/track.mp3",
  "spatial": "immersive",
  "targetLufs": -10,
  "final": { "integratedLufs": -11.7, "truePeakDb": -1.2, "lra": 1.1 },
  "monoSumLufs": -15.3,
  "monoDropDb": 3.6        // how much level is lost when summed to mono
}

Why does it land at -11.7 when I asked for -10? Linear loudnorm will back off the gain rather than exceed your true-peak ceiling. Lower truePeak (e.g. -2.0) gives the encoder more headroom; the driven limiter is what lets it reach loud targets without clipping.

export_dolby

Encode to a Dolby codec.

Param	Type	Default	Notes
input	string	—	mastered stereo file
output	string	—	.eac3 or .mp4
codec	"eac3" | "ac3"	"eac3"	E-AC-3 (DD+) or AC-3 (DD)
bitrate	string	"384k"
channels	number	2	2 stereo · 6 basic 5.1 upmix

---

Loudness targets

Use case	Integrated LUFS	True peak
Hip-hop / trap / "loud"	-7 to -10	-1 dBTP
Pop / rock	-9 to -11	-1 dBTP
Streaming reference	-14	-1 dBTP
Podcast / spoken	-16	-1.5 dBTP

Streaming platforms normalize to ~-14 LUFS, so a louder master is turned down on playback — louder ≠ "wins." Pick a target for the vibe, keep true peak at or below -1 dBTP.

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About "Dolby"

export_dolby produces channel-based Dolby codecs — the "Dolby" badge (E-AC-3 / AC-3). It is not object-based Dolby Atmos, and for a stereo music single it gives no sonic benefit over a good stereo master.

• Want a bigger sound? Use master_audio with spatial: "immersive".
• Want true Atmos (Apple Music Spatial Audio, etc.)? That requires Dolby's Atmos Renderer (built into Logic Pro) and your separate stems placed as 3D objects, exported as an ADM BWF master — a different pipeline that can't be done from ffmpeg.

The export_dolby tool description states this plainly so it's never oversold.

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Requirements

• Node.js ≥ 18
• ffmpeg on your PATH, built with these filters/encoders (all standard in modern ffmpeg): acrossover, aexciter, asoftclip, stereotools, extrastereo, acompressor, alimiter, loudnorm, and the eac3 / ac3 encoders.

Override the binary with the FFMPEG_PATH environment variable. Check your build:

ffmpeg -hide_banner -filters  | grep -E 'acrossover|aexciter|stereotools|alimiter|loudnorm'
ffmpeg -hide_banner -encoders | grep -E 'eac3|ac3'

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Troubleshooting

• loudnorm JSON not found — the input file path is wrong or ffmpeg isn't on PATH. Use absolute paths; set FFMPEG_PATH if needed.
• Master is quieter than targetLufs — the true-peak ceiling is capping the gain. Lower truePeak (more headroom) or accept that very dynamic material can't hit extreme loudness cleanly.
• immersive sounds too wide / weak in mono — check monoDropDb in the result; if it's high (> ~4 dB), use spatial: "subtle" instead.
• Output true peak slightly above target — MP3 encoding adds inter-sample peaks; target truePeak: -2.0 to land near -1.0 dBTP in the file.

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License

MIT © Ope Olatunji