Fade out music at the end of a video: two fades, two caps

To fade music out at the end of a video with Sume, set soundtrack.fade_out_seconds on a Timeline 1.0 render, up to 10 seconds. That fades only the music bed. A separate output.fade_out_seconds fades the whole render and is capped at 5 seconds.

The two fields look alike and have different caps, so pick by what you want to fade. Everything here is from Sume's Timeline 1.0 page, read 2026-10-01. YouTube's September 2026 post about conversational editing is the trend behind the question; the render itself is a plain API call.

What is the difference between the two fades?

How do I write the request?

Timeline needs an audio spine (audio.url, audio.parts[], or audio.mode: "silence") and video[] slots, all pointing at this workspace's media.sume.com files, plus an Idempotency-Key. The soundtrack block is separate from the spine. Note that duck_db needs a real spine, not silence.

{
  "audio": { "url": "https://media.sume.com/artifacts/artf_demo/voice.wav", "duration_seconds": 24 },
  "video": [
    { "source_url": "https://media.sume.com/artifacts/artf_demo/talk.mp4", "start": 0, "duration": 24 }
  ],
  "soundtrack": {
    "url": "https://media.sume.com/artifacts/artf_demo/bed.mp3",
    "gain_db": -12,
    "fade_out_seconds": 6
  },
  "output": { "fade_out_seconds": 1 }
}

Which one should I use?

If the voice or picture should stay at full level and only the music should taper, use the soundtrack fade. If you want the whole clip to ease out, including picture, use the output fade. They can be combined as above. Mind the sum rule: the output fades must add up to no more than the output length, so a 3-second clip cannot take a 5-second fade out.

A longer bed than the clip is handled with loop on the soundtrack. For the full background-music workflow see add background music to video; for the same idea in ffmpeg terms see ffmpeg fade in, fade out.

What does a render cost?

Timeline 1.0 is priced at $0.10 per output minute, and the reserve is ceil(audio.duration_seconds / 60) minutes. An unbilled /plan preflight returns the estimate before you submit.