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
• Smart True Peak ceiling: Based on AES TD1008, uses -0.5 dBTP for high-quality files, -1.0 dBTP for low-bitrate
• 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

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 bitrate-aware 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

Based on AES TD1008 recommendations. The ceiling depends on bitrate, not format:

Bitrate	Ceiling	Native lossless requires
Lossless (FLAC, AIFF, WAV)	-0.5 dBTP	—
Lossy ≥256kbps	-0.5 dBTP	TP ≤ -2.0 dBTP
Lossy <256kbps	-1.0 dBTP	TP ≤ -2.5 dBTP

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 v1.7.1            │
│   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, 1.5dB steps, target: -2.0 dBTP)
  Filename        LUFS    True Peak    Target        Gain
  track04.mp3    -14.0    -5.5 dBTP   -2.0 dBTP   +3.0 dB
  track05.mp3    -13.5    -6.0 dBTP   -2.0 dBTP   +3.0 dB

● 2 AAC/M4A files (native lossless, 1.5dB steps)
  Filename        LUFS    True Peak    Target        Gain
  track08.m4a    -13.0    -4.0 dBTP   -1.0 dBTP   +3.0 dB
  track09.m4a    -12.5    -4.5 dBTP   -1.0 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

✓ 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
Windows (Scoop)	scoop bucket add headroom https://github.com/M-Igashi/scoop-bucket && scoop install headroom
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

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)

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	-2.0	+3.5	mp3rgain	+3.0
track06.mp3	MP3	320	-12.0	-1.5	-0.5	+1.0	re-encode	+1.0
track08.m4a	AAC	256	-13.0	-4.0	-1.0	+3.0	native	+3.0
track10.m4a	AAC	256	-12.5	-1.8	-0.5	+1.3	re-encode	+1.3

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.

Bitrate-Aware Ceiling for Native Lossless

Since native lossless gain only works in 1.5dB steps, at least 1.5dB of headroom to the target ceiling is required:

• ≥256kbps: Target -0.5 dBTP → requires TP ≤ -2.0 dBTP
• <256kbps: Target -1.0 dBTP → requires TP ≤ -2.5 dBTP

Example: 320kbps file at -3.5 dBTP gets 2 steps (+3.0dB) → -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