← Skills

/stem-split

Separate a song or a film's soundtrack into stems (vocals, drums, bass, other) with Demucs, locally, on the Apple GPU when there is one. Use when user says or needs a voice separated from music in footage, stems for a mashup or sample chops, or drums+bass to detect music

install: npx skills add fortunto2/solo-factory/stem-split
source: fortunto2/solo-factory/tree/main/skills/stem-split
phase: utility

Any track becomes vocals.wav, drums.wav, bass.wav, other.wav (44.1 kHz stereo) with Meta’s htdemucs. Nothing is uploaded; the model downloads once.

# stem-split — Demucs stems, local

Any track becomes `vocals.wav`, `drums.wav`, `bass.wav`, `other.wav` (44.1 kHz stereo) with Meta's
`htdemucs`. Nothing is uploaded; the model downloads once.

## The command

```bash
uv tool install demucs --with numpy        # once; --with numpy avoids torch's "Failed to initialize NumPy"
demucs -d mps -o out "song.mp3"            # Apple Silicon GPU
demucs -o out "song.mp3"                   # CUDA if present, else CPU
```

No install: `uvx --with numpy --from demucs demucs -d mps -o out "song.mp3"` (`--from demucs`
because `uvx demucs` alone resolves the wrong entry point).

Output: `out/htdemucs/<file name without extension>/{vocals,drums,bass,other}.wav`.

Measured on an M-series Mac, 60 s of a song, 4 stems including model load: **8.3 s with `-d mps`,
15.7 s on CPU**, stems equal to 0.1 dB between the two. Without `-d`, Demucs does not pick the
Apple GPU on its own.

## Variants

```bash
demucs -d mps --two-stems=vocals -o out "song.mp3"   # vocals + no_vocals, ~2x faster: acapella or instrumental
demucs -d mps -n htdemucs_ft -o out "song.mp3"        # fine-tuned model, better, ~4x slower
demucs -d mps --mp3 --mp3-bitrate 320 -o out "song.mp3"
demucs -d mps -j 4 -o out "song.mp3"                  # more parallel jobs, more RAM
```

## Uses

- **Music:** stems for a mashup, a vocal for sample chops (no backing bleed), `no_vocals` as a clean
  instrumental bed or a filter loop.
- **Video, other people's music in the frame:** drums + bass is the music detector (engine, wind and
  city noise land in `other`, so "no_vocals" is blind to radio), and the **vocals** stem keeps the
  speaker when the music is replaced. Details: `/video-edit`, `references/sound.md`.
- **Video, a voice drowned by a live band:** raise the speaker from the vocals stem under one
  continuous span of the live audio.
- **Before audio-to-MIDI:** separate first; a pitch tracker on a full mix follows whatever is loudest.

For a film, extract the soundtrack first: `ffmpeg -i film.mp4 -vn -ac 2 -ar 44100 film_audio.wav`.

## Gotchas

- **The output folder can exist but be empty** if a run was killed. Check that all four `.wav`s are
  there before using them.
- Quote paths: Cyrillic and spaces work.
- Long files take minutes even on the GPU (~4 min per 20 min of audio); run them in the background.
- A singer's voice from a radio ends up in the vocals stem with the speaker's, 5–10 dB under. It
  disappears under a new score; it does not disappear on its own.
Sources