StemKit

Split any YouTube song into isolated stems — vocals, drums, bass, guitar, piano and more — right on your machine.

Search YouTube or paste a link, pick the instruments you want, and play the result like a mini DAW: the video on one side, every stem on its own fader, all perfectly in sync. Karaoke, acapellas and instrumentals are one click away.

Everything runs locally — no accounts, no cloud, no API keys.

Features

• Built-in YouTube search, or paste a link
• Choose your instruments individually; the right separation engine is picked for you
• Tight audio/video sync with instant, artifact-free seeking
• One-click presets: All · Karaoke · Acapella · Drums + Bass
• Per-stem mute/solo/volume, waveforms with click-to-seek
• Parallel background splitting with live progress
• Export any stem (or all) as WAV
• Fully offline after setup — separation runs on Apple Silicon (MPS), NVIDIA GPUs (CUDA) or CPU; ffmpeg included

Download

Grab installers from Releases:

• macOS (Apple Silicon): StemKit-x.y.z-mac-arm64.dmg
• Windows: StemKit-Setup-x.y.z.exe (installer) or portable .zip

First launch creates a private Python environment and downloads the separation engine (~2 GB) — one time. ffmpeg is bundled — nothing else to install.

Optional quality upgrades live behind a gear icon in the app (Settings), each with its own one-time download:

• Studio-quality vocals (Mel-Band Roformer): +913 MB — runs on GPU or CPU (CPU is slower)
• Fine-tuned demucs (htdemucs_ft): +~320 MB, up to 4× slower
• Refinement passes: 2 shifts instead of 1, up to 3× slower

macOS first launch: builds are signed with a Developer ID but not notarized, so macOS may say it "cannot verify the developer". One-time fix: System Settings → Privacy & Security → Open Anyway (or xattr -cr /Applications/StemKit.app).

Windows: SmartScreen may warn on first run — "More info → Run anyway".

Requirements

• macOS 12+ (Apple Silicon) or Windows 10/11 (x64)
• No manual installs: if no Python 3.9+ is detected, StemKit downloads a private runtime (python-build-standalone) during first-launch setup
• Node.js 20+ only for building from source

Develop

npm install
npm run dev

Wrong Node version? Scripts auto-relaunch with a suitable one (nvm / nvm-windows).

Build & release

bash scripts/fetch-ffmpeg.sh        # mac (one time)
powershell scripts/fetch-ffmpeg.ps1 # windows (one time)

npm run dist        # mac dmg -> release/
npm run dist:win    # windows nsis+zip -> release/
npm run dist:all    # both (on the matching OS)

Releases are built by GitHub Actions:

• push a tag v* → binaries attach to a draft GitHub Release
• workflow_dispatch ("Run workflow") → on-demand artifacts on the run page

macOS builds are Developer-ID-signed when the certificate is available — see Signing in CI below for the one-time setup, plus optional notarization.

Signing in CI (one-time setup)

Local builds sign with your keychain cert automatically. CI runners have empty keychains, so hand them the certificate via repo secrets:

1. Keychain Access → My Certificates → right-click Developer ID Application: ... → Export → .p12 (set an export password)
2. Base64 it and add these repo secrets:
   • CSC_MAC_P12 — the base64 string: base64 -i developer-id.p12 | pbcopy
   • CSC_MAC_PASSWORD — the export password from step 1
3. Optional (full notarization, zero Gatekeeper prompts): add APPLE_ID, APPLE_APP_SPECIFIC_PASSWORD, APPLE_TEAM_ID and set repo variable ENABLE_NOTARIZATION to true (Settings → Secrets and variables → Actions → Variables). Requires an active Apple Developer membership — Apple's notary service rejects expired accounts.

Without these secrets CI falls back to ad-hoc signing (app runs, but Gatekeeper complains on download).

How it works

YouTube URL ──► yt-dlp (+JS runtime) ──► bundled ffmpeg ──► mel-band roformer (vocals) ─┐
                                        │                                               ├─► stems/*.wav
                                        └─────────────► demucs htdemucs ────────────────┘
                                             (drums/bass/other, shift-averaged)

Electron renderer ◄──── IPC events ─────┘
video iframe (muted) + Web Audio stem playback · master clock = the audio itself

Notes

• Downloading audio from YouTube violates their ToS for public products — keep this personal.
• yt-dlp breaks occasionally when YouTube changes things; the error dialog offers a one-click update (updates yt-dlp + the challenge solver together).

Layout

src/main         Electron main process (pipeline, env bootstrap, library)
src/preload      IPC bridge
src/renderer     React UI (player, sync engine, waveforms)
python/          separate.py (demucs) and roformer.py (neural vocals) with JSON progress output
python/vendor/   patched model code — see python/vendor/README.md
scripts/         node runner, ffmpeg fetchers
build/           icon sources