Track selection and playback plans

Track selection chooses the actual audio and subtitle content for a source, affecting both user presentation and the media processing required by the playback path.

A source can contain several audio and subtitle streams. The viewer selects content, not just a codec: language, commentary, accessibility role and subtitle purpose can all matter.

Those choices also change what the playback path must handle.

A source with several English tracks

Use this original stream inventory:

Source-local index Role Language Representation
0 Video Not an audio-language choice H.264
1 Main audio English Six-channel AC-3
2 Main stereo audio English Stereo AAC
3 Main audio French Stereo AAC
4 Commentary English Stereo AAC
5 Full subtitles French SRT text
6 Forced subtitle material English PGS bitmap

Language alone cannot distinguish indexes 2 and 4. Their content roles differ. Channel count and codec information cannot establish that role either; descriptive evidence is needed.

Emby's playback request model includes audio and subtitle selection indexes alongside source context. Those coordinates refer to a particular representation.

Present is different from selected

An unsupported stream somewhere in a file is different evidence from an unsupported stream selected for the session. A player may ignore unselected tracks if its container path allows that.

Suppose the target accepts the video and stereo AAC but not the six-channel AC-3 presentation. Choosing index 1 calls for audio handling different from choosing index 2. The library item and video stream remain the same.

Similarly, selecting SRT that can be delivered as supported text differs from selecting bitmap material that the chosen path must render into video. The second choice can require replacing the video representation.

Flags supply clues with a scope

A default disposition can indicate an intended default track within the resource. It does not know every user's language or whether the user wants commentary.

A forced-subtitle flag describes intended use of the marked material. It does not guarantee that every player interprets the flag identically or that the stream's metadata is correct.

Remembered user choices and app policies supply another layer of selection. A diagnostic needs the actual chosen stream rather than guessing from the first listed track or a flag.

Changing source changes the coordinate system

Index 2 on another source can refer to different content. Output stream indexes can also change after mapping and repackaging.

A track choice should therefore be associated with source identity and checked against its stream description. Copying an index alone can turn “English main audio” into “English commentary” without causing a codec error.

For the original inventory, a test record should say source F1, audio index 2, French SRT index 5. “The movie works with English” leaves the selected role and subtitle path unresolved.

See Also

References