karaoke-video

Karaoke Video

Composite pipeline:

1. Extract audio from the source.
2. Run Demucs to separate stems (vocals, drums, bass, other).
3. Mix a karaoke audio bed: instrumental at full level + lead vocal at user-chosen attenuation (-12 dB default; mute removes entirely).
4. Build the lyric track:
   • If the user supplied an LRC or SRT, use it.
   • Otherwise, run whisper on the isolated vocals stem (much cleaner transcript than mixed audio) and convert to ASS with per-line timing.
5. Render the karaoke video — original picture + new audio bed + animated subtitles with music-note bullets.

Procedure

1. Resolve tools

DATA_DIR="${CLAUDE_USER_DATA:-${XDG_DATA_HOME:-$HOME/.local/share}/claude-plugins}/video-editing"
PREFS="$DATA_DIR/preferences.json"
VENV=$(jq -r '.python_venv // empty' "$PREFS" 2>/dev/null)

DEMUCS="$VENV/bin/demucs"
[ -x "$DEMUCS" ] || DEMUCS=$(command -v demucs)
[ -n "$DEMUCS" ] || { echo "Demucs not found. Run deps-setup and select demucs."; exit 1; }

Whisper backend resolution mirrors burn-subtitles (use preferences.subtitle_backend).

2. Inputs

Field	Default
Source	required (video or audio)
Vocal level	-12dB (alt: mute, -6dB, -18dB)
Lyrics	optional path to .lrc / .srt
Lyric style	bouncy (default) / block / word-highlight
Output	<basename>.karaoke.mp4

3. Demucs separation

WORK=$(mktemp -d)
"$DEMUCS" --two-stems=vocals -o "$WORK" "$SRC"
# Output layout: $WORK/htdemucs/<basename>/vocals.wav and no_vocals.wav
STEMS="$WORK/htdemucs/$(basename "${SRC%.*}")"
VOCALS="$STEMS/vocals.wav"
INSTR="$STEMS/no_vocals.wav"

For 4-stem separation (drums/bass/other), drop --two-stems=vocals. Two-stems is faster and sufficient for karaoke.

4. Mix the karaoke audio bed

case "$VOCAL_LEVEL" in
  mute)   FILTER="[1:a]anull[bed]" ;;                                # instrumental only
  *)      DB="${VOCAL_LEVEL/dB/}"; DB="${DB/-/-}"
          FILTER="[0:a]volume=${DB}dB[v];[v][1:a]amix=inputs=2:normalize=0:duration=longest[bed]" ;;
esac

ffmpeg -hide_banner -y -i "$VOCALS" -i "$INSTR" \
  -filter_complex "$FILTER" \
  -map "[bed]" -c:a aac -b:a 192k \
  "$WORK/karaoke-audio.m4a"

Set normalize=0 so the instrumental retains its loudness and the vocal is genuinely attenuated rather than auto-leveled back up.

5. Lyrics

If LRC supplied

LRC is line-timed ([mm:ss.xx]Lyric line). Convert to ASS with karaoke \k tags for per-syllable highlight (when lyric_style=word-highlight), or to plain SRT for block / bouncy.

For word-highlight: split each line into words, distribute the line's duration evenly across words, emit {\kNN}word tokens. (More accurate karaoke needs forced-alignment — out of scope.)

If SRT supplied

Use as-is for block / bouncy. For word-highlight, same word-split approach as LRC.

If nothing supplied — whisper the isolated vocal

"$WHISPER" --output_format srt --output_dir "$WORK" --language "$LANG" "$VOCALS"
LYRICS="$WORK/$(basename "${VOCALS%.*}").srt"

Vocals-only input gives whisper a much cleaner signal than the mix — output is significantly more accurate. Suggest the user run clean-transcription on this SRT before rendering.

6. Subtitle styling — music-note glyphs

ASS supports inline styling and Unicode glyphs. Build the karaoke ASS with a music-note prefix on each cue:

Style: Karaoke,Inter,52,&H00FFFFFF,&H000000FF,&H00000000,&HCC000000,-1,0,0,0,100,100,0,0,1,3,2,2,30,30,80,1
Dialogue: 0,0:00:05.00,0:00:08.50,Karaoke,,0,0,0,,♪ {\fad(200,200)}This is a lyric line ♪

For bouncy style, add a vertical wobble via \move or \t(0,500,\fscy105)\t(500,1000,\fscy100) (subtle 5% scale pulse on the beat — looks animated without forced-alignment data).

For word-highlight, use ASS karaoke tags (\k) so each word lights up in sequence as its duration elapses:

Dialogue: 0,0:00:05.00,0:00:08.50,Karaoke,,0,0,0,,♪ {\k50}This {\k40}is {\k30}a {\k60}lyric {\k70}line ♪

(The \k value is centiseconds. Computed by word_duration_ms / 10.)

Music-note glyphs to consider: ♪ ♫ ♬ 🎵. Stick to the BMP characters (♪♫♬) — emoji renders inconsistently with libass.

Save the styled file as $WORK/lyrics.ass.

7. Render

ESC_ASS=$(printf %s "$WORK/lyrics.ass" | sed -e 's/\\/\\\\/g' -e "s/'/\\\\'/g" -e 's/:/\\:/g')

ffmpeg -hide_banner -y \
  -i "$SRC" \
  -i "$WORK/karaoke-audio.m4a" \
  -filter_complex "[0:v]ass='${ESC_ASS}'[v]" \
  -map "[v]" -map 1:a \
  -c:v libx264 -crf 18 -preset medium \
  -c:a copy \
  -movflags +faststart \
  "$OUT"

The ass filter (vs subtitles) honours full ASS styling including animations.

For audio-only sources (no source video), pair this with audio-to-music-video to produce a waveform background, then run karaoke-video over that synthesised video.

8. Cleanup

rm -rf "$WORK"

…unless the user wants to keep the stems for further work — in which case copy to <project>/working/stems/ first.

9. Report

Source     : path/to/song.mp4
Stems      : separated via Demucs (htdemucs)
Vocals     : attenuated -12 dB (alt: mute / -6dB / -18dB)
Lyrics     : 42 lines (whisper-transcribed from isolated vocal)
Style      : word-highlight, ♪ glyphs, bouncy fade
Output     : path/to/song.karaoke.mp4 (1080p, 4:32, 38 MB)

Notes

• Demucs first run downloads the htdemucs model (~80 MB). Subsequent runs are fast.
• If the user wants a true sing-along (perfect per-syllable timing), forced alignment (e.g. aeneas, whisperx) is the right tool — out of scope here.
• Music-note glyph rendering depends on the system having a font that covers U+266A. Inter does. If a custom font is set in preferences.subtitle_style, verify it before rendering.
• Demucs is GPU-accelerated when CUDA is available. On AMD-only systems it runs CPU-only — separation of a 4-min track takes ~2–4 minutes.