heygen-com--hyperframes/skills/media-use/audio/references/bgm.md

Background music (BGM)

One music bed per composition, produced by the shared audio engine (scripts/audio.mjs → scripts/lib/bgm.mjs). Two routes, chosen by the engine's one switch — whether a HeyGen credential is present:

• HeyGen retrieval — the default when credentialed. Search HeyGen's music catalog by mood, download the top track. No generation; same ~/.heygen / $HEYGEN_API_KEY credential as TTS.
• Local generation (Lyria → MusicGen) — the fallback when there is no credential (or when asked for explicitly). Generate a WAV from a mood prompt. There is no npx hyperframes bgm command; the engine spawns scripts/lyria-recipe.py or an inline MusicGen script directly.

Run the Preflight first — no credential is not a green light to silently generate locally. Before generating, complete the sign-in Preflight (see ../SKILL.md → Preflight): run npx hyperframes auth status, recommend signing in, and STOP for the user's choice (sign in for HeyGen's music library, or continue offline with local generation). This applies to a one-off "generate a BGM" request just as much as inside a full workflow.

Driving it from the request

audio_request.json → bgm: { mode?, query?, prompt? }:

• mode — retrieve | generate | none. Omit for auto (retrieve when credentialed, else generate). An explicit retrieve is strict: no credential ⇒ skip, never a detached generate (so a caller with no wait-bgm step, e.g. product-launch, can't get a pending job it won't await).
• query — the mood, used for retrieval and as a fallback prompt seed (e.g. a storyboard's music: field, falling back to message → arc → "calm cinematic underscore").
• prompt — an explicit full prompt for generation; omit and the engine infers one (see Mood inference). Optional blob / archetype / arc feed that inference.

HeyGen retrieval (default)

searchSounds(query, "music", { limit: 5 }) → GET /audio/sounds?query=<mood>&type=music&limit=5. Take the top result (ranked by score), download its presigned audio_url → assets/bgm/track.mp3. Synchronous. No match → skip (BGM is optional; never fail the render over it). Cue written to audio_meta.json:

{
  "path": "assets/bgm/track.mp3",
  "volume": 0.12,
  "mode": "retrieve",
  "query": "calm cinematic underscore",
  "duration_s": 42.0,
}

volume comes from the engine's bgmDefaultVolume(): BGM_BED_VOLUME (currently 0.12 ≈ -18 dB — a bed under the voice) under narration, BGM_SILENT_VOLUME (currently 0.9) for a silent film (no voice). Tune those constants in scripts/lib/bgm.mjs, not call sites. An explicit volume in audio_meta.json always overrides this default. bgm_pending is false — the file is on disk when the engine returns.

Local generation (fallback) — Lyria → MusicGen

Spawned detached so voice work isn't blocked; audio_meta.bgm_pending: true and bgm_pid / bgm_log are set until it finishes. Run scripts/wait-bgm.mjs before assembling — it polls the output file / process / log, detects crashes, and writes bgm_status.json (status: ready | failed | timeout | disabled). A failed/absent track is simply omitted; it never blocks voice/SFX.

Order	Provider	Env / deps	Speed	Quality
1	Google Lyria RealTime	$GEMINI_API_KEY or $GOOGLE_API_KEY + google-genai (auto-installed on demand)	Real-time stream (≈ requested duration)	Production-grade
2	MusicGen (facebook/musicgen-small)	Python transformers + torch + soundfile + numpy (~300 MB first run; auto-installed)	Slow on CPU; fast on Apple MPS / CUDA	Decent; prompt-only control

Output → assets/bgm/track.wav, target = total voice duration. MusicGen generates one seed clip (≤28–30s, under the decoder's positional limit) then crossfade-loops it up to the target (or trims down if shorter), avoiding per-segment seams. Backend selection is by what can actually run: Lyria only when import google.genai succeeds, else MusicGen; if neither can be made to run, BGM is skipped (voice + SFX still render).

Mood inference (the generate prompt)

inferBgmPrompt() in scripts/lib/bgm.mjs: an explicit prompt wins; otherwise industry-keyword base → narrative-archetype shape → emotional-arc tiebreaker.

Match in blob / query	Base prompt	BPM
crypto / nft / web3 / defi / token / blockchain	atmospheric electronic, deep bass, futuristic synths, restrained percussion	100
finance / fintech / bank / payment / invest / wealth	calm cinematic, soft strings, subtle piano, restrained percussion	92
creative / agency / design / studio / art / brand	playful electronic, warm pads, light percussion	115
(default: SaaS / tech / platform)	uplifting corporate tech, bright modern piano with synth pads	108

Archetype then reshapes the arc — PAS → "MINOR to MAJOR" build; BAB / future-pacing → aspirational rising; feature-cascade → +10 BPM driving; demo-loop → −8 BPM minimal. The emotional arc breaks remaining ties (tension→relief, excitement, trust/reassurance).

Lyria knobs (direct recipe use)

The engine bakes BPM / scale into the prompt text (via the inference above) and passes only --output / --duration / --prompt to the recipe. If you invoke scripts/lyria-recipe.py directly you can also set: --bpm (90–110 calm, 110–130 energetic), --brightness (0–1, ≥0.7 promotional), --density (0–1, higher = fuller), --scale (MAJOR / MINOR / PENTATONIC / …), --negative-prompt (styles to exclude). MusicGen ignores all of these — put the mood in the prompt.

Failure modes

Failure	Behavior
No music match (retrieve)	bgm: null, anomaly logged. Render proceeds without BGM.
Explicit retrieve, no credential	Skipped (no silent generate fallback). Use mode: generate or omit mode for auto.
Neither Lyria nor MusicGen can run (generate)	bgm disabled with a pip install … hint. Voice + SFX still render.
Generate still rendering at assemble time	bgm_pending: true; wait-bgm.mjs waits/checks and writes bgm_status.json first.
Generate crashed	wait-bgm.mjs → bgm_status.json { status: "failed" }; the <audio> track is omitted.

BGM failure never blocks a render.