Audio separation

Two bun scripts in ~/.config/opencode/skills/audio-separation/. Both take a local file or a URL, upload local files to fal storage, and save the results next to the input (or in --out). They print a JSON map of saved paths to stdout and progress to stderr.

S=~/.config/opencode/skills/audio-separation
bun $S/demucs.ts song.mp3                          # song.vocals.wav + song.instrumental.wav
bun $S/sam.ts interview.mp3 --prompt 'man speaking' # interview.target.wav + interview.residual.wav

Which script

goal	script	model	price (3 min audio)
song vocals vs instrumental, karaoke, acapella	demucs.ts	fal-ai/demucs htdemucs_ft	~$0.13
drums, bass, other stems	demucs.ts --stems	same	~$0.13
guitar and piano stems too	demucs.ts --model htdemucs_6s --stems	htdemucs_6s	~$0.13
speech over noise, dialogue from video, any sound by text	sam.ts --prompt '...'	fal-ai/sam-audio/separate	~$0.30
a sound you can locate in time	sam.ts --span 6-10	fal-ai/sam-audio/span-separate	~$0.30

Default to demucs.ts for music. It is cheaper and trained on music stems. Use sam.ts when the target is not a standard music stem, or when Demucs leaves too much bleed and the user accepts the higher price.

API key

FAL_KEY lives in .env next to the scripts (gitignored). The scripts read it themselves; an exported FAL_KEY wins. If it is empty the script exits with an error: ask the user to fill it from https://fal.ai/dashboard/keys. Never print the key.

demucs.ts

bun $S/demucs.ts <file-or-url> [--out dir] [--model htdemucs_ft] [--format wav|mp3] [--stems] [--shifts 1-10]

• instrumental is the sum of the non-vocal stems, mixed locally with ffmpeg.
• --stems keeps the separate drums/bass/other files.
• --shifts above 1 improves quality a little but runs longer.

sam.ts

bun $S/sam.ts <file-or-url> --prompt 'what to isolate' [--predict-spans] [--candidates 1-7]
  [--acceleration fast|balanced|quality] [--span 6-10 ...] [--exclude-span 20-22 ...] [--trim]
  [--format wav|mp3] [--out dir]

• target is the prompted sound, residual is everything else.
• Write short concrete prompts: man speaking, female singing voice, dog barking.
• --candidates 3 reranks multiple outputs. Better quality, +50% cost per extra candidate.
• --span / --exclude-span switch to the span endpoint. --trim cuts output to the spans.
• fal documents WAV, MP3 and FLAC input. For video, extract audio first: ffmpeg -i in.mp4 -vn -b:a 192k in.mp3.

Discord

After a run, upload the outputs with kimaki upload-to-discord --session <current-session> <files...>. Convert to mp3 first if wav files exceed Discord upload limits (ffmpeg -i x.wav -b:a 192k x.mp3).