Music bed shorter than the voiceover: loop, fade and duck in Timeline

When a Music 1.0 bed ends before the voiceover, set loop on the Timeline soundtrack. Timeline will repeat the bed to the length of the audio spine, apply a fade-out of up to 10 seconds, and duck the bed by 0 to 20 dB under the narration. The spine is your voiceover; the soundtrack is the music.

The soundtrack fields

Timeline 1.0 takes one audio spine (your narration, up to 1,800 seconds) plus an optional soundtrack bed. The bed sits under the spine. These are the soundtrack options the docs list.

Loop or regenerate

Music 1.0 is a flat $0.125 per generation regardless of length, so there is no price reason to loop a short bed. The reason is control: there is no duration parameter, you steer length with the prompt ('a 2-minute track'), and the result can come back shorter or longer than you asked. A 3-minute narration with a 2-minute bed is a loop, not a failure.

If you want a bed that never repeats, write section markers such as [0:00-0:30] Intro in the prompt and ask for the length you need. If a repeat is fine, loop and save a retry.

Errors to expect

Timeline rejects a fade longer than the output with soundtrack_fade_exceeds_output, and duck_db on a silent spine with duck_requires_audio_spine. The plan call (unbilled) runs the same schema and compiler checks, so run it before you render. The render is $0.10 per ceil(output minute): a 3-minute narration is $0.30, and the bed adds the single $0.125 generation.

A starting point for speech

A bed under narration usually needs a gentle level. Start with duck_db around 10 and a fade_out_seconds of 4, then listen. These are starting values to tune by ear, not Sume recommendations. Keep the prompt for the bed instrumental, with no vocals and no spoken word, so it does not fight the voice.

Cost of the whole stack

For a 3-minute narrated clip: narration of about 2,700 characters at the planning assumption of 900 characters a minute is 2,700 x 47.5 = 128,250 micro-dollars, 13 cents; one bed is $0.125; the render is 3 x $0.10 = $0.30. Total about $0.555. Looping the bed adds nothing, since loop is a render setting.

The planning figure of 900 characters a minute is an assumption for this estimate, not a Sume number.