Label tracks with roles

A track’s role is its author-declared purpose — main audio, director’s commentary, audio description, closed captions, forced narrative subtitles, and so on. dyndo records the role in the descriptor and renders it into both the DASH and HLS manifests, so players present the track correctly: which audio a viewer hears by default, which subtitles auto-enable, and which renditions are flagged for accessibility.

Roles apply to audio and text tracks only — never video.

Set a role while indexing

A role is never probed from the media; you declare it. Add role=<role> to a track descriptor when you index it:

dyndo index \
  video.mp4 \
  audio_en.mp4,language=eng,role=main \
  audio_en_commentary.mp4,language=eng,role=commentary \
  -o asset.json

To set a role on a track that’s already in the descriptor, re-index it — index merges by source path, so this updates the entry in place and changes nothing else about it:

dyndo index audio_en_commentary.mp4,role=commentary -o asset.json

You can also hand-edit the role field in asset.json directly; it takes the same values.

Audio roles

Role	Use it for	Effect on players
`main`	The primary audio.	Becomes the default audio rendition.
`alternate`	An alternate mix of the main audio.	Auto-selectable (e.g. to match the viewer’s language).
`dub`	A dubbed rendition in another language.	Auto-selectable (e.g. to match the viewer’s language).
`commentary`	Commentary, e.g. director’s.	Opt-in: never auto-selected, the viewer chooses it.
`description`	Audio description for blind / low-vision viewers.	Opt-in, and flagged as accessible audio description.
`enhanced-audio-intelligibility`	Dialogue enhanced for intelligibility.	Opt-in, and flagged as an accessibility rendition.

The default audio rendition is the first track you mark main; if you mark none, it’s the first audio track. Mark exactly one track main to control what plays by default.

Text (subtitle) roles

Text-track manifest output is still being completed — today a text role shows up in DASH (for CMAF wvtt sources); HLS subtitle renditions land next. See Track roles for the exact per-protocol state.

Role	Use it for	Effect on players
`subtitle`	Translation subtitles (dialogue only).	Off by default; the viewer turns them on. This is also the default when no role is set.
`caption`	SDH / closed captions (dialogue plus non-dialogue sound).	Off by default; flagged with the SDH accessibility characteristics.
`forced-subtitle`	Forced narrative (foreign dialogue, on-screen text).	Auto-enabled and marked `FORCED`, so it shows even when subtitles are otherwise off.

Examples

A main track plus a commentary track — the viewer hears the main audio by default and can switch to commentary:

dyndo index \
  video.mp4 \
  audio_en.mp4,language=eng,role=main \
  audio_en_comm.mp4,language=eng,role=commentary \
  -o asset.json

An audio-description track for accessibility:

dyndo index audio_en_ad.mp4,language=eng,role=description -o asset.json

Forced subtitles that display automatically for foreign-language dialogue — set the role directly on the .vtt you index:

dyndo index forced_en.vtt,language=eng,role=forced-subtitle -o asset.json

What gets rejected

index validates roles as it reads each descriptor and aborts the whole run on:

a role (or language) on a video input;
a role that doesn’t apply to the track’s media type — e.g. subtitle on audio, or main on text; or
an unknown role value.

Next steps

The exact DASH and HLS output for every role: Track roles reference.
How role sits in the descriptor: asset.json descriptor.
Where roles are set: Index your CMAF sources.

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