The Open Karaoke Format

One file format. Karaoke + DJ software. No vendor lock-in.

What is Stem MP4?

Stem MP4 is an open audio format that stores multiple audio tracks (called "stems") in a single file. Originally created by Native Instruments for DJs, it lets you control individual parts of a song — like vocals, drums, bass, and instruments — independently.

Loukai extends this format with karaoke features: synchronized lyrics with word-level timing. Because it's built on standard MP4, your karaoke files work everywhere.

📄
song.stem.mp4
🎵 Master (full mix)
🥁 Drums
🎸 Bass
🎹 Other
🎤 Vocals
📝 Lyrics + Timing

Why Open Format Matters

🔓

No Vendor Lock-in

Your karaoke library isn't trapped in one app. Move your files anywhere, use any software that supports M4A.

🎧

Works with DJ Software

The same files you use for karaoke work in Traktor and Mixxx with full stem control.

📱

Universal Playback

Any music player can play M4A files. Share songs with friends — they don't need special software.

💾

Smaller Files

AAC compression means smaller files than uncompressed formats, without sacrificing audio quality.

Use Your Karaoke Files for DJing

Same files, different purpose. No conversion needed.

Native Instruments Traktor

Full stem support. Control each stem's volume, apply effects, and create live remixes.

Learn more →

Mixxx

Free and open source DJ software with full stem support. Control individual tracks in real-time.

Learn more →

Note: DJ software will play the audio and read standard metadata (title, artist, BPM, key). The karaoke-specific data (lyrics) is stored in custom atoms that DJ software simply ignores — it doesn't cause any problems.

How to Create Stem MP4 Files

Process your music locally with AI

1

Open Loukai Creator

Go to the Creator tab in the Loukai app.

→
2

Drop Your Audio

Drag your MP3, FLAC, or WAV file.

→
3

AI Does the Work

Demucs separates stems, Whisper transcribes lyrics.

→
4

Ready to Use

Your .stem.mp4 file works in Loukai and DJ apps.

Download Loukai Free

Technical Details

For developers and the curious

File Structure

  • Container: Standard MPEG-4 (.stem.mp4)
  • Audio: 5 AAC tracks (master + drums, bass, other, vocals)
  • Metadata: Standard MP4 atoms (title, artist, key, BPM)
  • Karaoke data: Custom atom (kara)

Audio Tracks (NI Stems order)

  • Track 0: Master (original mix)
  • Track 1: Drums
  • Track 2: Bass
  • Track 3: Other (instruments, melody)
  • Track 4: Vocals

Karaoke Extensions

  • kara: Lyrics with word-level timing
  • stem: NI Stems metadata for DJ compatibility
  • Real-time pitch: Detected during playback

AI Tools Used

  • Demucs: Meta's stem separation model
  • Whisper: OpenAI's speech recognition
  • CREPE: Key detection algorithm

For Developers

  • stem-mp4: JS/Node helper library to read & write Stem MP4 files (FFmpeg-free extraction, works in browsers too)
  • Live demo: Play a .stem.mp4 in the browser and mute/solo stems
  • m4aplayer: Source code for the demo (React + Web Audio API)

Want the full specification?

View Full Spec on GitHub stem-mp4 Helper Library

Format Comparison

Feature Stem MP4 (Loukai) CDG Proprietary Karaoke
Open standard ✅ Yes ✅ Yes (but limited) ❌ No
Stem separation ✅ 4 stems ❌ No ❌ Usually not
Word-level lyrics ✅ Yes ❌ No ✅ Sometimes
DJ software compatible ✅ Yes ❌ No ❌ No
Single file ✅ Yes ❌ MP3 + CDG pair ✅ Usually
Create your own ✅ Free with Loukai ❌ Requires special tools ❌ Subscription services

Ready to Try the Open Format?

Download Loukai and start creating your own karaoke files today.

Download Loukai Free