headroom 2.0.0

Audio loudness analyzer and gain adjustment tool for mastering workflows
headroom-2.0.0 is not a library.

headroom

A toolkit for Rekordbox DJ workflows: loudness normalization for CDJ export, plus a Rekordbox XML playlist sorter for harmonic mixing.

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.

New in v2.0.0 — a companion rbsort subcommand sorts a Rekordbox playlist by Camelot Key (1A→12B) then BPM ascending, and appends the result as a new playlist to your collection.xml. Useful for harmonic mixing prep when the Rekordbox UI does not expose multi-column sort. See Rekordbox Playlist Sorter.

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)
  • Rekordbox playlist sorter (v2.0+): headroom rbsort produces a new playlist sorted by Camelot Key then BPM

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 v2.0.0            │
│   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.

Rekordbox Playlist Sorter (rbsort)

Added in v2.0.0.

Rekordbox does not expose a "sort by Key AND BPM" option in its UI. headroom rbsort reads your collection.xml, sorts each target playlist by Camelot Key (1A → 12B) ascending then BPM ascending, and emits the sorted copies into a new Sorted (Key+BPM)/ folder appended to the same XML. Originals are left untouched. The layout mirrors headroom's backup/ directory: one container folder, each item inside keeps its source name.

This is the same idea as headroom's analyzer applied to playlist order: Rekordbox's software-only features (Auto Gain, multi-column sort) don't follow your tracks to the CDJ. rbsort bakes Key+BPM order into the playlist itself — so when you export to USB in Rekordbox's EXPORT mode, the CDJ plays the set in that exact order with no on-deck reordering.

Workflow

  1. Set key display to Alphanumeric (1A..12B notation) in Rekordbox: Preferences > View > Key display format > Alphanumeric.
  2. Export: File > Export Collection in xml format → e.g. ~/Music/rekordbox/collection.xml.
  3. Run rbsort: 
    # Sort every TrackID-referenced playlist in the XML
headroom rbsort --xml ~/Music/rekordbox/collection.xml

# Or target one playlist (top-level: just the name)
headroom rbsort \
  --xml ~/Music/rekordbox/collection.xml \
  --playlist "Happy House and Trance"

# Or target one nested under a folder
headroom rbsort \
  --xml ~/Music/rekordbox/collection.xml \
  --playlist "Sets/Friday"
    Output defaults to collection-out.xml next to the input. Pass --output <PATH> to override.
  4. Point Rekordbox at the output XML: Preferences > Advanced > Database > rekordbox xml > Imported Library → select collection-out.xml, then restart Rekordbox (Rekordbox only re-reads the XML on startup).
  5. Open the rekordbox xml tree in the left sidebar. It is a separate tree from your main library — switch to it from the sidebar icon column on the far left (the icon labeled rekordbox xml). Inside you'll find a new folder Sorted (Key+BPM)/ containing each sorted playlist under its original name.
  6. Verify the sort by clicking any playlist inside Sorted (Key+BPM)/ — tracks should run 1A (lowest BPM) → 1B → 2A → … → 12B (highest BPM).
  7. Drag the playlists you want from Sorted (Key+BPM)/ into your main Playlists collection. Your original playlists (still in Playlists) are unchanged.
  8. Export to USB for CDJ: switch Rekordbox to EXPORT mode (top-left dropdown), plug in your USB / SD, then right-click the playlist → Export Playlist. CDJs read tracks in playlist order by default — your Key+BPM sort plays back on the deck in that exact order.

The sorted results live only inside the rekordbox xml > Sorted (Key+BPM)/ folder, not in your main Playlists. If you only see the originals, you're looking at the local library — switch sidebar trees.

Flags

Flag Description
--xml <PATH> Path to collection.xml (required)
--playlist <PATH> Source playlist under the Rekordbox Playlists root. Optional — if omitted, every TrackID-referenced playlist in the XML is sorted. Top-level playlists: just the name (e.g. "Happy House and Trance"). Nested: /-separate folder/playlist names (e.g. "Folder/SubFolder/MyPlaylist")
--output <PATH> (-o) Output XML path. Optional — defaults to <input-stem>-out.<ext> next to the input
--name <NAME> Override the sorted playlist's name. Only valid with --playlist. When sorting all playlists, each sorted copy reuses its source name

Sort rules

  • Primary: Camelot Key ascending — 1A → 1B → 2A → 2B → … → 12A → 12B
  • Secondary: BPM ascending within each key group
  • Tracks with no Camelot key sort after all known keys; within a key group, tracks with BPM 0 / unanalyzed sort last

Notes

  • Requires the Tonality field to be exported as 1A..12B (Rekordbox's "Alphanumeric" key display format). Non-matching values (e.g. Am, C#) are silently sorted last.
  • Only KeyType="0" (TrackID-referenced) playlists are supported. In all-playlists mode, non-KeyType=0 playlists are silently skipped; for a single target, rbsort errors out.
  • rbsort does not require ffmpeg — only the analyzer subcommand does.
  • A single Sorted (Key+BPM)/ folder is appended inside the <PLAYLISTS> ROOT NODE, regardless of how many playlists were processed. The ROOT Count is bumped by 1.

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