demix

audio processing tool: download from YouTube or use local files, separate stems, adjust tempo/pitch, detect and transpose key, and cut segments.

prerequisites

[!NOTE] Please note: I'm using homebrew for installing virtualenvwrapper and ffmpeg. If you're using another package manager or different operating system than macOS (e.g. Linux), you need to install it differently. In the examples for creating virtualenv below, replace python path below with your own python3.8 path. I'm personally using pyenv for installing various python verions, but you can do it as you wish.

installation

I suggest to create virtualenv for this project to not break existing system-wide installations:

brew install virtualenvwrapper
brew install ffmpeg
brew install yt-dlp
mkdir demix
cd demix
mkvirtualenv -p /Users/pw/.pyenv/versions/3.8.16/bin/python demix
workon demix
pip install demix
demix -v

update

pip install demix --upgrade

development

prepare environment:

brew install virtualenvwrapper
brew install ffmpeg
git clone git@github.com:pwittchen/demix.git
cd demix
mkvirtualenv -p /Users/pw/.pyenv/versions/3.8.16/bin/python demix
workon demix
pip install -r requirements.txt
python demix.py -v

virtualenv

exit virtualenv, when you're done:

deactivate

to activate env again:

workon demix

testing

install pytest:

pip install pytest

run all tests (-v param for verbose):

pytest -v

versioning and deployment

When we create and push a new git tag, e.g. v1.0.4, deploy.yml github action is triggered. It automatically extracts created tag, updates version with bump_version.py script, performs git commit and push. After that, deployment of the new package version to PyPi is executed.

usage

demix -u <youtube-url> [options]
demix -s <search-query> [options]
demix -f <audio-file> [options]

options

modes

examples

# separate a YouTube video into vocals and accompaniment
demix -u 'https://www.youtube.com/watch?v=VIDEO_ID' -m 2stems

# search YouTube by artist and song name
demix -s 'Queen - Bohemian Rhapsody' -m 4stems

# separate a local file with 4 stems
demix -f /path/to/song.mp3 -m 4stems

# cut audio from 1:30 to 3:45
demix -f song.mp3 -ss 1:30 -to 3:45

# start from 0:30 (skip intro)
demix -f song.mp3 -ss 0:30

# keep only the first 2 minutes
demix -f song.mp3 -to 2:00

# combine cutting with tempo and transpose
demix -f song.mp3 -ss 1:00 -to 4:00 -t 0.8 -p -2

# detect musical key of a song
demix -f song.mp3 -k

# detect key before and after transposing
demix -f song.mp3 -k -p -3

# transpose to a specific key (auto-detects current key)
demix -f song.mp3 -K C

# transpose to A minor
demix -f song.mp3 -K Am

# transpose to target key with tempo change
demix -f song.mp3 -K "F# minor" -t 0.9

# slow down without separating stems
demix -f song.mp3 -t 0.8 -m nosplit

# download and cut without separation
demix -u 'https://www.youtube.com/watch?v=VIDEO_ID' -ss 1:00 -to 3:00 -m nosplit

# run silently and only print the final result message
demix -f song.mp3 -m 4stems -q

youtube downloads

When given -u or -s, demix downloads audio with yt-dlp if it's on PATH (brew install yt-dlp), falling back to pytubefix otherwise.

YouTube periodically restricts which player clients may fetch media without a PO token, and a blocked client typically fails with HTTP Error 403: Forbidden partway through the download. To survive that, demix retries the download across several yt-dlp player clients before giving up:

If every strategy fails, demix reports what each attempt returned instead of a single opaque error.

passing extra yt-dlp options

Set DEMIX_YT_DLP_ARGS to append arbitrary arguments to every yt-dlp invocation. This is the usual remedy when downloads are blocked and you're logged into YouTube in a browser:

DEMIX_YT_DLP_ARGS='--cookies-from-browser chrome' demix -u 'https://www.youtube.com/watch?v=VIDEO_ID' -m 2stems

Other useful values: --proxy http://…, --limit-rate 1M, --sleep-requests 2. If downloads still fail, brew upgrade yt-dlp often restores access, since new releases track YouTube's changes closely.

MCP server

The repo also ships an MCP server at mcp/ so LLM clients (Claude Desktop, Claude Code, etc.) can call demix directly. It's a sibling Python package (demix-mcp, Python 3.10+) that shells out to the demix CLI on PATH — keeping it decoupled from demix's own Python 3.8 environment.

Install (in a separate Python 3.10+ env):

pipx install ./mcp

Claude Code

Register the server with the claude mcp CLI (run from your demix workspace so pretrained_models/ and output/ cache there):

claude mcp add-json demix '{"command":"demix-mcp","cwd":"'"$PWD"'"}'

Add --scope user to make it available across all projects, or --scope project to commit it into .mcp.json for your team. Verify with:

claude mcp list

Inside Claude Code, run /mcp to inspect server status and the exposed tools.

Claude Desktop / other clients

Add the server to ~/Library/Application Support/Claude/claude_desktop_config.json (or your client's equivalent):

{
  "mcpServers": {
    "demix": {
      "command": "demix-mcp",
      "cwd": "/path/to/demix-workspace"
    }
  }
}

Tools exposed: process_audio, detect_key, search_youtube, clean. See mcp/README.md for details.

Claude Code skill

This repo ships a Claude Code skill at .claude/skills/demix/SKILL.md that translates plain-English requests into demix invocations — describe what you want and Claude picks the flags, shows the command, and runs it.

Examples of requests the skill handles:

• "Download https://youtu.be/abc123 and give me the instrumental" → demix -u 'https://youtu.be/abc123' -m 2stems
• "Find 'Radiohead Creep', slow it 15%, keep only the first two minutes" → demix -s 'Radiohead Creep' -t 0.85 -to 2:00
• "~/Music/song.mp3 — transpose to G minor and split into 4 stems" → demix -f '~/Music/song.mp3' -m 4stems -K 'Gm'
• "What key is song.wav in?" → demix -f 'song.wav' -k
• "Make a karaoke video from 'Adele - Hello'" → demix -s 'Adele - Hello' -m 2stems --video

Trigger it by typing /demix in Claude Code, or just describe the task in natural language — the skill loads automatically when the intent matches.