AutoMixer 🎵

Automatic DJ-style audio mixer that sequentially blends MP3 files with BPM detection, pitch adjustment, and beat synchronization.

Features

• 🎯 Automatic BPM Detection - Analyzes audio files to detect tempo using beat detection algorithms
• 🔄 Beat Synchronization - Aligns beats between tracks for seamless transitions
• 🎚️ Pitch-Preserving Tempo Adjustment - Adjusts tempo while maintaining original pitch
• 🌊 Smooth Crossfades - Creates professional equal-power crossfades between tracks
• 📊 Audio Analysis - Extracts metadata, duration, and audio characteristics
• 🖥️ CLI & API - Use from command line or integrate into your Node.js projects

Prerequisites

• Node.js >= 18.0.0
• FFmpeg with FFprobe (required for audio processing)

Installing FFmpeg

# macOS
brew install ffmpeg

# Ubuntu/Debian
sudo apt install ffmpeg

# Windows (with Chocolatey)
choco install ffmpeg

# Windows (with winget)
winget install FFmpeg

Installation

Global Installation (CLI)

npm install -g automixer

Local Installation (Library)

npm install automixer

CLI Usage

Mix Multiple Tracks

# Basic usage - mix tracks in order
automixer mix track1.mp3 track2.mp3 track3.mp3 -o my_mix.mp3

# Specify crossfade duration (default: 8 seconds)
automixer mix track1.mp3 track2.mp3 -c 12 -o output.mp3

# Set a specific target BPM
automixer mix track1.mp3 track2.mp3 -b 128 -o output.mp3

# Allow pitch to change with tempo (faster processing)
automixer mix track1.mp3 track2.mp3 --no-preserve-pitch -o output.mp3

Analyze Tracks

# Analyze a single track
automixer analyze track.mp3

# Analyze multiple tracks
automixer analyze track1.mp3 track2.mp3

# Output as JSON
automixer analyze track.mp3 --json

Check System Requirements

automixer check

CLI Options

Option	Description	Default
-o, --output <file>	Output file path	mix_output.mp3
-c, --crossfade <seconds>	Crossfade duration	8
-b, --bpm <number>	Target BPM	Auto-detect
--max-bpm-change <percent>	Maximum BPM change allowed	8
--no-preserve-pitch	Allow pitch to change with tempo	Pitch preserved
-q, --quiet	Suppress progress output	Show progress

API Usage

Basic Example

import AutoMixer from 'automixer';

const mixer = new AutoMixer({
  crossfadeDuration: 8,    // seconds
  preservePitch: true,
  maxBPMChange: 8          // percent
});

// Mix multiple tracks
await mixer.mix(
  ['track1.mp3', 'track2.mp3', 'track3.mp3'],
  'output.mp3'
);

Advanced Usage with Events

import { AutoMixer } from 'automixer';

const mixer = new AutoMixer({
  crossfadeDuration: 10,
  targetBPM: 128,
  preservePitch: true
});

// Listen to events
mixer.on('analysis:track:start', ({ index, filepath }) => {
  console.log(`Analyzing track ${index + 1}: ${filepath}`);
});

mixer.on('analysis:track:complete', ({ index, trackInfo }) => {
  console.log(`Track ${index + 1}: ${trackInfo.bpm} BPM`);
});

mixer.on('mix:bpm', ({ targetBPM }) => {
  console.log(`Target BPM: ${targetBPM}`);
});

mixer.on('mix:render:progress', ({ current, total, message }) => {
  console.log(`Progress: ${current}/${total}`);
});

mixer.on('mix:complete', ({ outputPath }) => {
  console.log(`Mix saved to: ${outputPath}`);
});

// Run the mix
await mixer.mix(['track1.mp3', 'track2.mp3'], 'output.mp3');

Step-by-Step Processing

import { AutoMixer } from 'automixer';

const mixer = new AutoMixer();

// Add tracks
mixer.addTracks(['track1.mp3', 'track2.mp3', 'track3.mp3']);

// Analyze tracks
const analyzedTracks = await mixer.analyzeTracks();

// Log track information
for (const track of analyzedTracks) {
  console.log(`${track.filename}: ${track.bpm} BPM, ${track.duration}s`);
}

// Get optimal BPM
const targetBPM = mixer.calculateOptimalBPM();
console.log(`Optimal BPM: ${targetBPM}`);

// Create the mix
await mixer.createMix('output.mp3');

Using Individual Components

import { BPMDetector, AudioAnalyzer, PitchShifter } from 'automixer';

// Detect BPM
const detector = new BPMDetector();
const { bpm, beats, confidence } = await detector.detect('track.mp3');
console.log(`BPM: ${bpm} (confidence: ${confidence})`);

// Get audio metadata
const analyzer = new AudioAnalyzer();
const metadata = await analyzer.getMetadata('track.mp3');
console.log(`Duration: ${metadata.duration}s`);

// Adjust tempo
const shifter = new PitchShifter();
const adjustedPath = await shifter.adjustTempo('track.mp3', 1.1, true);
console.log(`Tempo-adjusted file: ${adjustedPath}`);

API Reference

AutoMixer

Main class that orchestrates the mixing process.

Constructor Options

Option	Type	Default	Description
crossfadeDuration	number	8	Crossfade duration in seconds
targetBPM	number	null	Target BPM (auto-detect if null)
preservePitch	boolean	true	Preserve pitch when changing tempo
maxBPMChange	number	8	Maximum BPM change percentage
outputFormat	string	'mp3'	Output format
outputBitrate	number	320	Output bitrate in kbps

Methods

• addTracks(filepaths) - Add tracks to the queue
• clearTracks() - Clear all tracks
• analyzeTracks() - Analyze all tracks (returns track info)
• calculateOptimalBPM() - Calculate the optimal target BPM
• createMix(outputPath) - Create the final mix
• mix(inputFiles, outputPath) - Full process in one call

Events

• analysis:start - Analysis started
• analysis:track:start - Individual track analysis started
• analysis:track:complete - Individual track analysis completed
• analysis:complete - All analysis completed
• mix:start - Mixing started
• mix:bpm - Target BPM calculated
• mix:prepare:start - Track preparation started
• mix:prepare:complete - Track preparation completed
• mix:render:start - Rendering started
• mix:render:progress - Rendering progress update
• mix:complete - Mixing completed

BPMDetector

Detects BPM and beat positions in audio files.

const detector = new BPMDetector({ minBPM: 60, maxBPM: 200 });
const { bpm, beats, confidence } = await detector.detect('track.mp3');

AudioAnalyzer

Extracts metadata and analyzes audio characteristics.

const analyzer = new AudioAnalyzer();
const metadata = await analyzer.getMetadata('track.mp3');
// { duration, sampleRate, channels, bitrate, codec, format, tags }

PitchShifter

Adjusts tempo and pitch of audio files.

const shifter = new PitchShifter();

// Adjust tempo (preserving pitch)
await shifter.adjustTempo('input.mp3', 1.1, true);

// Shift pitch (in semitones)
await shifter.shiftPitch('input.mp3', 2);

// Adjust both
await shifter.adjustTempoAndPitch('input.mp3', 1.1, 2);

TrackMixer

Handles crossfading and track mixing.

const mixer = new TrackMixer({
  crossfadeDuration: 8,
  crossfadeCurve: 'log' // 'linear', 'log', 'sqrt', 'sine', 'exponential'
});

How It Works

BPM Detection

AutoMixer uses the music-tempo library combined with FFmpeg for BPM detection:

1. Audio is decoded to raw PCM samples using FFmpeg
2. A 30-second analysis window is extracted (skipping intro)
3. Beat detection algorithm identifies tempo and beat positions
4. BPM is normalized to a standard range (60-200)
5. Beats are extrapolated across the full track

Beat Matching

The mixing algorithm:

1. Analyzes all input tracks to detect BPM
2. Calculates optimal target BPM (median of all tracks)
3. Adjusts each track's tempo to match target BPM
4. Finds beat-aligned transition points
5. Creates equal-power crossfades at transition points

Tempo Adjustment

Tempo adjustment uses FFmpeg's audio filters:

• With pitch preservation: Uses the rubberband filter for high-quality time-stretching
• Without pitch preservation: Uses the atempo filter for simple speed changes

Performance Tips

• Use consistent BPM tracks: Mixing tracks with similar BPMs produces better results
• Allow larger BPM changes: Increase maxBPMChange if tracks have very different tempos
• Longer crossfades: Use longer crossfades (10-15s) for smoother transitions
• Skip pitch preservation: Use --no-preserve-pitch for faster processing when pitch shift is acceptable

Troubleshooting

FFmpeg Not Found

Make sure FFmpeg is installed and available in your PATH:

ffmpeg -version

Poor BPM Detection

Some tracks may have ambiguous tempos. You can:

• Set a specific target BPM with -b option
• Increase maxBPMChange to allow larger adjustments

Audio Quality Issues

• Ensure source files are high quality
• Use higher bitrate: outputBitrate: 320
• Use pitch-preserved tempo adjustment

License

MIT License - see LICENSE file for details.

Repository

Project repository: https://github.com/manalejandro/automixer

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

1. Fork the repository
2. Create your feature branch (git checkout -b feature/amazing-feature)
3. Commit your changes (git commit -m 'Add amazing feature')
4. Push to the branch (git push origin feature/amazing-feature)
5. Open a Pull Request

Changelog

v1.0.0

• Initial release
• BPM detection and beat synchronization
• Pitch-preserving tempo adjustment
• Smooth crossfade mixing
• CLI and API interfaces