Uncompose

Open-source music source separation.

Uncompose separates recorded music into vocals, drums, bass, and instruments — locally, openly, and without a subscription. It is a local-first tool for musicians who want to isolate or reduce parts of a recording for practice, transcription, remixing, or study, while keeping control over their audio, models, and workflow.

Status

Pre-v0.1: the first release is being built in the open on the issue tracker, with decisions recorded in docs/adr/. The install lines below go live with the v0.1.0 tag.

Install

Uncompose ships on PyPI with uv as the recommended installer — uv is also what Uncompose itself uses at runtime, so have it installed either way:

uvx uncompose separate song.mp3        # run without installing
uv tool install uncompose              # or install the `uncompose` command

The package is small; the heavy part comes on first use. Uncompose then builds its engine environment (PyTorch and the separation stack, several GB, CUDA build by default) into ~/.local/share/uncompose/ — announced with progress, one time per version. On a machine without an NVIDIA GPU, make that first run route torch to the CPU build:

UV_TORCH_BACKEND=cpu uncompose separate song.mp3

Later runs reuse the environment; the variable is only needed while it is first built. Model weights (≈900 MB for the default preset) download automatically on first use, hash-verified, with each model's license status shown.

Quickstart

uncompose separate song.mp3

That produces song.stems/ next to the input:

song.stems/
├── vocals.wav  drums.wav  bass.wav  guitar.wav  keys.wav  other.wav
├── engine.log
└── job.json

Every run prints a header first — input, preset, models with their license status, device, output folder — so you know what is about to happen before the slow part starts. Repeated runs on the same input never overwrite: you get song.stems-2/, song.stems-3/, and so on. job.json records everything needed to understand or rerun the job (input hash, models, device, timings); it is written last, so its presence means the folder is complete.

Then:

uncompose play vocals      # audition a stem of the last job (mpv or ffplay)
uncompose open             # open the last job's folder
uncompose models list      # cached models, license status, hardware needs
uncompose models fetch 6-stem   # pre-download weights before a session

Presets and devices

• 6-stem (default): vocals, drums, bass, guitar, keys, other. A Mel-Band RoFormer vocal pass followed by htdemucs_6s for the rest.
• 2-stem (--preset 2-stem): vocals and instrumental. GPU-required.

The keys.wav caveat: the v0.1 model behind it is piano-trained. Acoustic piano lands in keys.wav; synths and organs usually land in other.wav. The stem keeps the name keys so nothing changes when a broader keys model arrives.

Device is auto-detected: CUDA when an NVIDIA GPU is present, CPU otherwise (--device cpu|cuda overrides). On the machine of record a song takes 1 to 5 minutes on GPU; CPU produces identical stems but takes tens of minutes — correct, just slow.

Requirements

Uncompose is Linux-only for v0.1: NVIDIA CUDA is the primary target with CPU as the slow-but-correct fallback, and the machine of record is Ubuntu Studio 26.04 with an RTX 4060 Ti. macOS and Windows are untested; reports welcome. It needs ffmpeg on your PATH to read and write audio. Install it with your system package manager, for example sudo apt install ffmpeg on Debian/Ubuntu. If ffmpeg is missing, Uncompose stops before a run with a one-line install message rather than a cryptic error.

Responsible use

Uncompose processes audio you provide, entirely on your own machine — nothing is uploaded anywhere. You are responsible for making sure you have the rights to the audio you separate, and the rights to what you do with the resulting stems follow from the rights you hold in the input. Separating a recording does not grant you any rights to it.

The separation models Uncompose can download carry their own licenses, some of which restrict commercial use; Uncompose surfaces each model's license status but it is up to you to comply with it.

License

MIT © Dominic Hanzely