A 28-second bed under a 59-second Short: loop, duck, $0.225
Timeline soundtrack loop repeats a 28-second bed under a 59-second voice spine, with a fade-out up to 10 s and duck up to 20 dB. Render $0.10 plus Music $0.125.
Set the soundtrack's loop flag and a 28-second bed repeats under a 59-second voice spine: two full passes (56 seconds) and 3 seconds of a third. Add fade_out_seconds (up to 10) so the last pass does not stop abruptly, and duck_db (0 to 20) to push the bed down under speech. If the bed is a fresh Music 1.0 generation at $0.125 flat, and the render is 59 seconds, which bills 1 minute at $0.10, the whole Short is $0.225 as of 2026-10-08.
What the soundtrack object accepts
The Timeline docs list the soundtrack fields: a url, a gain_db, a loop flag, fade_out_seconds up to 10, and duck_db from 0 to 20. Ducking needs a real audio spine. With audio.mode set to silence the request is refused with duck_requires_audio_spine, because there is no voice to duck under.
| Field | Value used | Limit from the docs | Effect |
|---|---|---|---|
| audio.duration_seconds | 59 | 1 to 1800 | Output length, and the billed minutes: 1 |
| soundtrack.loop | true | Boolean | 28 s bed repeats: 59 / 28 = 2.1 passes |
| soundtrack.fade_out_seconds | 4 | Up to 10 | Bed fades over the last 4 s |
| soundtrack.duck_db | 12 | 0 to 20 | Bed sits 12 dB lower under the spine |
| soundtrack.gain_db | -3 | Optional | Overall bed level |
The cost, line by line
The render is the same $0.10 for any length from 1 to 60 seconds, because Timeline bills started output minutes. The bed is the only variable. A Music 1.0 generation is $0.125 flat; a bed you already own costs $0.00 on Sume. So the range for this Short is $0.10 to $0.225.
- Render: ceil(59 / 60) = 1 minute x $0.10 = $0.10
- Bed from Music 1.0: $0.125 flat per generation
- Total with a generated bed: $0.10 + $0.125 = $0.225
- Total with an owned bed: $0.10
Why 59 seconds, and what the loop hides
The YouTube Help page says a Short over one minute with an active copyright claim is blocked globally, and one under a minute sees no change in limitations. Staying at 59 seconds keeps you under that line. It does not remove the claim risk of the bed itself. The page says royalty-free tracks from the YouTube Audio Library will not get a Content ID claim, and it says nothing of the sort for other sources, including generated music.
A loop is audible. At 28 seconds the seam falls at 28 and 56 seconds, and a bed with a clear start and end will announce it. Pick a bed that ends on a held note or fades, or lengthen it by choosing a longer bed. If the seam still shows, a fade_out_seconds of a second or two on the bed's own file is cheaper than regenerating.
Check before you render
Send the same body to the unbilled plan endpoint. It returns billable_minutes (1 here) and the estimated cost for the render, without spending a credit. It cannot see the sound of the seam, so listen to the result once. If you need the longer bed for a 3-minute Short instead, the same loop flag covers it, and the render becomes ceil(180 / 60) = 3 minutes, $0.30.
Sources
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Written by Sume