headroom 1.10.1

Audio loudness analyzer and gain adjustment tool for mastering workflows
headroom-1.10.1 is not a library.

headroom

Audio loudness analyzer and gain adjustment tool for mastering and DJ workflows.

What is this?

headroom simulates the behavior of Rekordbox's Auto Gain feature, but with a key difference: it identifies files with available headroom (True Peak below the target ceiling) and applies gain adjustment without using a limiter.

This tool is designed for DJs and producers who want to maximize loudness while preserving dynamics, ensuring tracks hit the optimal True Peak ceiling without clipping.

Key Features

  • Single binary: mp3rgain is built-in as a library — only ffmpeg required as external dependency
  • Uniform True Peak ceiling: -0.5 dBTP for every file by default — the most aggressive, AES TD1008–blessed delivery target — fully overridable via --tp-target
  • Multiple processing methods: ffmpeg for lossless formats, built-in mp3rgain for lossless MP3/AAC gain, ffmpeg re-encode for precise gain
  • Non-destructive workflow: Original files are backed up before processing
  • Metadata preservation: Audio tags (ID3v2, Vorbis comment, BWF) are preserved during processing, and files are overwritten in place so Rekordbox cue points and other external metadata remain linked
  • No limiter: Pure gain adjustment only — dynamics are preserved
  • Interactive CLI: Guided step-by-step process with two-stage confirmation
  • Scriptable CLI: Non-interactive mode for pipelines and CI (paths, globs, and flags)

Processing Methods

headroom selects the optimal method for each file based on format and headroom:

Format Method Precision Quality Loss
FLAC, AIFF, WAV ffmpeg Arbitrary None
MP3, AAC/M4A mp3rgain (built-in) 1.5dB steps None (global_gain modification)
MP3, AAC/M4A ffmpeg re-encode Arbitrary Inaudible at ≥256kbps

Three-Tier Approach for Lossy Formats (MP3/AAC)

Each MP3 and AAC/M4A file is categorized into one of three tiers:

  1. Native Lossless — ≥1.5 dB headroom to the configured ceiling

    • Truly lossless global_gain header modification in 1.5dB steps
    • Uses built-in mp3rgain library
    • Applied automatically (no user confirmation needed)

  2. Re-encode — headroom exists but <1.5 dB to ceiling

    • Uses ffmpeg for arbitrary precision gain
    • MP3: libmp3lame with -q:a 0 / AAC: libfdk_aac (falls back to built-in aac)
    • Preserves original bitrate; requires explicit user confirmation

  3. Skip — no headroom available

True Peak Ceiling

Default — uniform delivery target

Every file targets -0.5 dBTP by default. This is the maximum-aggression value that AES TD1008 §7B describes for high-rate codec inputs ("may work satisfactorily with as little as -0.5 dBTP for the limiting threshold").

File class Ceiling Native lossless requires
Lossless (FLAC, AIFF, WAV) -0.5 dBTP —
MP3 (any bitrate) -0.5 dBTP TP ≤ -2.0 dBTP
AAC/M4A (any bitrate) -0.5 dBTP TP ≤ -2.0 dBTP

Why a single ceiling — pre-encode vs delivery

TD1008 has two related but distinct numbers:

  1. Generic delivery recommendation (§4) — "Maximum True Peak level not exceed -1 dBTP at the codec input of lossy-encoded streams." This is the pre-encode limiter threshold.
  2. High-rate codec relaxation (§7B) — "High-rate (e.g., 256 kbps) coders may work satisfactorily with as little as -0.5 dBTP" — also a codec-input threshold; "the limiting threshold may need to be reduced below the recommended -1.0 dBTP" for lower bit rates.

Both bullets describe the limiter that sits in front of the encoder. headroom operates in the opposite position: on already-encoded delivery files. There is no further codec stage downstream to absorb additional overshoot, so the bitrate-dependent slack TD1008 grants the pre-encode limiter does not transfer to the end product. A single, codec-agnostic delivery ceiling is the correct interpretation. -0.5 dBTP is chosen because it is the most aggressive value TD1008 sanctions for any limiter in the chain; lossless and high-rate lossy files were already at -0.5, and low-rate files now stop giving up an unnecessary 0.5 dB of loudness.

See docs/true-peak-ceiling.md for a longer walk-through with citations.

Tuning the ceiling

Goal Flag Resulting ceiling
Default (max-aggressive delivery) (none) -0.5 dBTP for all files
Match Spotify / Apple Music / YouTube delivery max --tp-target -1.0 -1.0 dBTP for all files
Conservative master with extra player headroom --tp-target -2.0 -2.0 dBTP for all files
Mirror TD1008's pre-encode interpretation --tp-split-bitrate -0.5 dBTP ≥256 kbps, -1.0 dBTP <256 kbps

--tp-target and --tp-split-bitrate are mutually exclusive. --tp-split-bitrate reproduces headroom's pre-1.10 default exactly.

The native-lossless threshold scales with the chosen ceiling: it is always target − 1.5 dB (e.g. -0.5 → TP ≤ -2.0; -1.0 → TP ≤ -2.5; -2.0 → TP ≤ -3.5).

How It Works

  1. Scans the current directory for audio files (FLAC, AIFF, WAV, MP3, AAC/M4A)
  2. Measures LUFS (Integrated Loudness) and True Peak using ffmpeg
  3. Categorizes files by processing method:
    • Green: Lossless files (ffmpeg)
    • Yellow: MP3/AAC files with enough headroom for native lossless gain
    • Magenta: MP3/AAC files requiring re-encode
  4. Displays categorized report
  5. Two-stage confirmation:
    • First: "Apply lossless gain adjustment?" (lossless + native MP3/AAC)
    • Second: "Also process files with re-encoding?" (MP3/AAC requiring re-encode)
  6. Creates backups and processes files

Example

$ cd ~/Music/DJ-Tracks
$ headroom

╭─────────────────────────────────────╮
│          headroom v1.7.3            │
│   Audio Loudness Analyzer & Gain    │
╰─────────────────────────────────────╯

▸ Target directory: /Users/xxx/Music/DJ-Tracks

✓ Found 28 audio files
✓ Analyzed 28 files

● 3 lossless files (ffmpeg, precise gain)
  Filename        LUFS    True Peak    Target        Gain
  track01.flac   -13.3    -3.2 dBTP   -0.5 dBTP   +2.7 dB
  track02.aif    -14.1    -4.5 dBTP   -0.5 dBTP   +4.0 dB
  track03.wav    -12.5    -2.8 dBTP   -0.5 dBTP   +2.3 dB

● 2 MP3 files (native lossless, 1.5 dB steps, requires TP ≤ -2.0 dBTP)
  Filename        LUFS    True Peak    Target        Gain
  track04.mp3    -14.0    -5.5 dBTP   -0.5 dBTP   +4.5 dB
  track05.mp3    -13.5    -6.0 dBTP   -0.5 dBTP   +4.5 dB

● 2 AAC/M4A files (native lossless, 1.5 dB steps, requires TP ≤ -2.0 dBTP)
  Filename        LUFS    True Peak    Target        Gain
  track08.m4a    -13.0    -4.0 dBTP   -0.5 dBTP   +3.0 dB
  track09.m4a    -12.5    -4.5 dBTP   -0.5 dBTP   +3.0 dB

● 2 MP3 files (re-encode required for precise gain)
  Filename        LUFS    True Peak    Target        Gain
  track06.mp3    -12.0    -1.5 dBTP   -0.5 dBTP   +1.0 dB
  track07.mp3    -11.5    -1.2 dBTP   -0.5 dBTP   +0.7 dB

● 1 AAC/M4A files (re-encode required)
  Filename        LUFS    True Peak    Target        Gain
  track10.m4a    -12.5    -1.8 dBTP   -0.5 dBTP   +1.3 dB

▸ TP target: -0.5 dBTP (uniform delivery ceiling, AES TD1008 §7B)

✓ Report saved: ./headroom_report_20250109_123456.csv

? Apply lossless gain adjustment to 3 lossless + 2 MP3 (lossless gain) + 2 AAC/M4A (lossless gain) files? [y/N] y

ℹ 2 MP3 + 1 AAC/M4A files have headroom but require re-encoding for precise gain.
  • Re-encoding causes minor quality loss (inaudible at 256kbps+)
  • Original bitrate will be preserved
? Also process these files with re-encoding? [y/N] y

? Create backup before processing? [Y/n] y
✓ Backup directory: ./backup

✓ Done! 10 files processed.
  • 3 lossless files (ffmpeg)
  • 2 MP3 files (native, lossless)
  • 2 AAC/M4A files (native, lossless)
  • 2 MP3 files (re-encoded)
  • 1 AAC/M4A files (re-encoded)

Installation

headroom requires ffmpeg. Package managers install it automatically.

Platform Command
macOS (Homebrew) brew install M-Igashi/tap/headroom
Windows (winget) winget install M-Igashi.headroom
Arch Linux (AUR) yay -S headroom-bin
Cargo cargo install headroom (ffmpeg must be installed separately)

Pre-built binaries are available on the Releases page (ffmpeg must be installed separately).

Build from Source

git clone https://github.com/M-Igashi/headroom.git
cd headroom
cargo build --release

Usage

Interactive Mode

Run without arguments to use the guided workflow in the current directory:

cd ~/Music/DJ-Tracks
headroom

The tool will guide you through:

  1. Scanning and analyzing all audio files
  2. Reviewing the categorized report
  3. Confirming lossless processing
  4. Optionally enabling MP3/AAC re-encoding
  5. Creating backups (recommended)

Scriptable Mode

Pass paths, globs, or flags to run non-interactively (useful for pipelines and scripts):

# Analyze a directory without modifying anything
headroom --analyze-only ~/Music/DJ-Tracks

# Apply only lossless gain, with backup, save report to a specific path
headroom --lossless --backup ./bak --report results.csv ./album/

# Enable re-encoding as well
headroom --lossless --reencode --backup ./bak ./album/

# Operate on specific files
headroom --lossless track1.mp3 track2.flac

# Glob patterns
headroom --lossless --no-report "./music/**/*.mp3"

# Tighter ceiling for streaming-platform delivery (Spotify / Apple / YouTube max)
headroom --lossless --tp-target -1.0 ./album/

# Restore the legacy bitrate-dependent split (pre-v1.10 behaviour)
headroom --lossless --tp-split-bitrate ./album/

Non-interactive defaults (when any flag or path is provided):

  • --lossless is on unless --no-lossless
  • --reencode is off unless --reencode is explicitly passed
  • --backup is off unless provided; bare --backup uses <target>/backup
  • CSV report is written unless --no-report; --report PATH sets a custom location
  • --analyze-only runs analysis + report only, skips processing

Run headroom --help for the full flag reference.

Output

CSV Report

Filename Format Bitrate (kbps) LUFS True Peak (dBTP) Target (dBTP) Headroom (dB) Method Effective Gain (dB)
track01.flac Lossless - -13.3 -3.2 -0.5 +2.7 ffmpeg +2.7
track04.mp3 MP3 320 -14.0 -5.5 -0.5 +5.0 mp3rgain +4.5
track06.mp3 MP3 320 -12.0 -1.5 -0.5 +1.0 re-encode +1.0
track08.m4a AAC 256 -13.0 -4.0 -0.5 +3.5 native +3.0
track10.m4a AAC 256 -12.5 -1.8 -0.5 +0.7 re-encode +0.7

Backup Structure

./
├── track01.flac             ← Modified
├── track04.mp3              ← Modified
├── track08.m4a              ← Modified
├── subfolder/
│   └── track06.mp3          ← Modified
└── backup/                  ← Created by headroom
    ├── track01.flac         ← Original
    ├── track04.mp3          ← Original
    ├── track08.m4a          ← Original
    └── subfolder/
        └── track06.mp3      ← Original

Important Notes

  • Files are overwritten in place after backup — Rekordbox metadata remains linked
  • Only files with positive effective gain are shown and processed
  • MP3/AAC native lossless requires at least 1.5dB headroom to be processed
  • MP3/AAC re-encoding is opt-in and requires explicit confirmation
  • macOS resource fork files (._*) are automatically ignored

Technical Details

Why 1.5dB Steps?

Both MP3 and AAC store a "global_gain" value as an integer. Each ±1 increment changes the gain by 2^(1/4) = ±1.5 dB. This is a format-level constraint, not a tool limitation.

headroom uses the built-in mp3rgain library to directly modify this field — no decoding or re-encoding involved.

Native Lossless Threshold

Since native lossless gain only works in 1.5 dB steps, at least 1.5 dB of headroom to the configured target ceiling is required. The threshold scales automatically:

Target Requires TP ≤
-0.5 dBTP (default) -2.0 dBTP
-1.0 dBTP (--tp-target -1.0) -2.5 dBTP
-2.0 dBTP (--tp-target -2.0) -3.5 dBTP

Example: 320 kbps file at -3.5 dBTP, default target → 2 steps (+3.0 dB) → -0.5 dBTP (optimal).

Re-encode Quality

At ≥256kbps, re-encoding introduces quantization noise below -90dB — far below audible threshold. Only gain is applied (no EQ, compression, or dynamics processing), and original bitrate is preserved.

License

MIT