Loop background music under a long video with Timeline 1.0

Set soundtrack.loop to true. In a Sume Timeline 1.0 render, a soundtrack that is shorter than the audio spine stops partway through unless it loops, and the render warns soundtrack_shorter_than_spine. With loop on, the bed repeats until the spine ends. Add fade_out_seconds so the last repeat does not stop dead.

The soundtrack fields

Steps

• Generate or upload a bed. Music from the music router costs $0.125 per accepted generation.
• Call POST /v1/timeline-1.0/plan with your request. It is unbilled, and returns the plan. It cannot predict soundtrack_shorter_than_spine, because that warning needs the bed's probed length, which is known only at render time.
• Render with POST /v1/timeline-1.0/render, at $0.10 per output minute.
• Listen at the seam. A bed that loops mid-phrase will click or lurch, so choose a bed with an even bar structure.

Choosing numbers

Leave gain_db near its -16 default when there is a voice spine. Use duck_db around 6 to 12 for dense speech, then adjust by ear. The duck follows the spine envelope, so it is a target depth against speech at normal level, not a constant. Ducking is rejected with a silent audio mode (duck_requires_audio_spine), because there is nothing to key from. Fades on the whole output use output.fade_in_seconds and fade_out_seconds, each 0 to 5.

Limits

Audio duration is 1 to 1,800 seconds. Looping repeats the file as is; it does not crossfade or beat-match, so a seamless loop depends on the music itself. Ask the music prompt for an arc that resolves where it starts. Read the full field list in Timeline 1.0.