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, ffmpeg re-encode for precise MP3/AAC gain
• Non-destructive workflow: Original files are backed up before processing
• Metadata preservation: Files are overwritten in place, so Rekordbox tags, cue points, and other metadata remain intact
• No limiter: Pure gain adjustment only — dynamics are preserved
• Interactive CLI: Guided step-by-step process with two-stage confirmation

Supported Formats & Processing Methods

MP3 Processing: Three-Tier Approach

headroom intelligently chooses the best method for each MP3 file:

1. Native Lossless (Built-in mp3rgain, bitrate-aware ceiling)

For MP3 files with ≥1.5 dB headroom to bitrate-aware ceiling:

• Truly lossless global_gain header modification
• 1.5 dB step increments (MP3 format specification)
• Uses built-in mp3rgain library
• ≥256kbps: -0.5 dBTP ceiling (requires TP ≤ -2.0 dBTP)
• <256kbps: -1.0 dBTP ceiling (requires TP ≤ -2.5 dBTP)

2. Re-encode (Precise, bitrate-aware ceiling)

For MP3 files with headroom but <1.5 dB to ceiling:

• Uses ffmpeg for arbitrary precision gain
• Preserves original bitrate
• Requires explicit user confirmation

3. Skip (No headroom)

Files already at or above the target ceiling are not processed.

AAC/M4A Processing

AAC/M4A files are supported with the following characteristics:

• Always requires re-encoding: Unlike MP3, AAC has no lossless gain adjustment mechanism
• Bitrate-aware ceiling: Same strategy as MP3 re-encode (-0.5 dBTP for ≥256kbps, -1.0 dBTP for lower)
• High-quality encoder: Prefers libfdk_aac when available, falls back to built-in aac encoder
• Preserves original bitrate: Maintains audio quality matching the source file
• Grouped with MP3 re-encode: AAC files are processed together with MP3 re-encode files in the second confirmation stage

Why Re-encode AAC is Safe at High Bitrates

Same principles apply as MP3 re-encoding:

• At ≥256kbps, quantization noise stays below -90dB (inaudible)
• Only gain is applied (no EQ, compression, or dynamics processing)
• Original bitrate is preserved
• Requires explicit user opt-in

True Peak Ceiling Strategy

Based on AES TD1008 recommendations:

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 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)
   • 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 v1.4.1            │
│   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 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

● 2 AAC/M4A files (re-encode required)
  Filename        LUFS    True Peak    Target        Gain
  track08.m4a    -13.0    -2.5 dBTP   -0.5 dBTP   +2.0 dB
  track09.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) files? [y/N] y

ℹ 2 MP3 + 2 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! 9 files processed.
  • 3 lossless files (ffmpeg)
  • 2 MP3 files (native, lossless)
  • 2 MP3 files (re-encoded)
  • 2 AAC/M4A files (re-encoded)

Installation

Quick Install

Prerequisites

headroom requires one external tool:

• ffmpeg: For audio analysis and lossless format processing

Note: As of v1.3.0, mp3rgain is built-in as a library dependency. No separate installation required. v1.4.1 uses mp3rgain 1.4.0.

macOS (Homebrew) — Recommended

brew install M-Igashi/tap/headroom

ffmpeg is installed automatically as a dependency.

Windows (Scoop)

scoop bucket add headroom https://github.com/M-Igashi/scoop-bucket
scoop install headroom

ffmpeg is installed automatically as a dependency.

Cargo (All Platforms)

If you have Rust installed, you can install headroom via cargo:

cargo install headroom

Then install ffmpeg for your platform:

# macOS
brew install ffmpeg

# Ubuntu/Debian
sudo apt install ffmpeg

# Fedora
sudo dnf install ffmpeg

# Arch
sudo pacman -S ffmpeg

# Windows (winget)
winget install ffmpeg

# Windows (choco)
choco install ffmpeg

Pre-built Binaries

Download pre-built binaries from the Releases page:

Note: You must install ffmpeg separately (see platform-specific commands above).

Build from Source

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

# Binary location:
# - Unix: target/release/headroom
# - Windows: target\release\headroom.exe

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

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 native lossless requires at least 1.5dB headroom to -2.0 dBTP 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 for Native MP3 Gain?

The MP3 format stores a "global_gain" value as an 8-bit integer (0-255). When decoding, samples are multiplied by 2^(gain/4):

• +1 to global_gain = 2^(1/4) = +1.5 dB
• -1 to global_gain = 2^(-1/4) = -1.5 dB

This is a fundamental limitation of the MP3 format, not a tool limitation. headroom uses the built-in mp3rgain library to directly manipulate this field in each MP3 frame's side information.

Why Bitrate-Aware Ceiling for Native MP3?

With 1.5dB step limitation, the ceiling is calculated based on bitrate to match re-encode targets:

• ≥256kbps: Target -0.5 dBTP, so native lossless requires TP ≤ -2.0 dBTP (allowing at least 1 step)
• <256kbps: Target -1.0 dBTP, so native lossless requires TP ≤ -2.5 dBTP (more conservative)
• Example: 320kbps file at -3.5 dBTP gets 2 steps (+3.0dB) → -0.5 dBTP (optimal)
• Example: 128kbps file at -3.5 dBTP gets 1 step (+1.5dB) → -2.0 dBTP (within -1.0 ceiling)

Why AAC Always Requires Re-encoding

Unlike MP3, AAC doesn't have a lossless gain adjustment mechanism like the global_gain field. The only way to apply gain to AAC files is through re-encoding. However, at high bitrates (≥256kbps), the quality loss is imperceptible.

MP3/AAC Re-encode Quality

When re-encoding is chosen:

• MP3: Uses libmp3lame encoder with -q:a 0 (best VBR quality)
• AAC: Prefers libfdk_aac (highest quality), falls back to built-in aac encoder
• Preserves original bitrate
• Only applies volume filter (no other processing)

At 320kbps, the re-encode introduces quantization noise below -90dB—far below audible threshold.

Processing Method Comparison

License

MIT