Timeline audio concat fails on mono plus stereo: channel mismatch

If you concat a mono file with a stereo file in Sume timeline audio, the worker returns audio_parts_channel_mismatch. The docs state that all parts must have the same channel layout. The join is sample-domain with no re-synthesis, so Sume does not convert layouts for you. Make the layouts match before you join.

Where mismatches come from

Narration recorded or generated as mono and music generated as stereo is the common case. Parts taken from different sources, such as a voice file and the audio track of a video, can also differ. The check is made by the worker, not at admit, so the error appears after the job is accepted.

Fix with audio detach

Audio detach has a channels option: source (default) or mono. If your parts come from videos, detach each with channels: "mono" and they all share a layout. Detach is $0.01 per job, so normalizing three parts costs 3 x $0.01 = $0.03, plus $0.01 for the concat, $0.04 in total.

If the parts are already audio files, the docs offer no channel conversion in timeline audio. Regenerate or re-detach the odd file in the layout the others use.

Request that works

Both parts below are assumed to be mono wav files already in your workspace.

curl -X POST https://api.sume.com/v1/timeline-1.0/audio \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: timeline-audio-concat-002" \
  -d '{
    "operation": "concat",
    "parts": [
      { "url": "https://media.sume.com/artifacts/artf_demo/line1.wav" },
      { "url": "https://media.sume.com/artifacts/artf_demo/line2.wav" }
    ]
  }'

After the join

The result has one audio_url, a duration_seconds and segments[] with the offsets of each part. Use the offsets to re-base your video slots in a Timeline 1.0 render.