97 lines
4.3 KiB
97 lines
4.3 KiB
4 years ago
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# Multiple Alternative Audio Tracks
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## General
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m3u8 manifests with multiple audio streams will have those streams added to `video.js` in an `AudioTrackList`. The `AudioTrackList` can be accessed using `player.audioTracks()` or `tech.audioTracks()`.
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## Mapping m3u8 metadata to AudioTracks
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The mapping between `AudioTrack` and the parsed m3u8 file is fairly straight forward. The table below shows the mapping
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| m3u8 | AudioTrack |
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|---------|------------|
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| label | label |
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| lang | language |
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| default | enabled |
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| ??? | kind |
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| ??? | id |
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As you can see m3u8's do not have a property for `AudioTrack.id`, which means that we let `video.js` randomly generate the id for `AudioTrack`s. This will have no real impact on any part of the system as we do not use the `id` anywhere.
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The other property that does not have a mapping in the m3u8 is `AudioTrack.kind`. It was decided that we would set the `kind` to `main` when `default` is set to `true` and in other cases we set it to `alternative` unless the track has `characteristics` which include `public.accessibility.describes-video`, in which case we set it to `main-desc` (note that this `kind` indicates that the track is a mix of the main track and description, so it can be played *instead* of the main track; a track with kind `description` *only* has the description, not the main track).
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Below is a basic example of a mapping
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m3u8 layout
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``` JavaScript
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{
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'media-group-1': [{
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'audio-track-1': {
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default: true,
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lang: 'eng'
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},
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'audio-track-2': {
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default: false,
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lang: 'fr'
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},
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'audio-track-3': {
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default: false,
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lang: 'eng',
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characteristics: 'public.accessibility.describes-video'
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}
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}]
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}
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```
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Corresponding AudioTrackList when media-group-1 is used (before any tracks have been changed)
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``` JavaScript
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[{
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label: 'audio-tracks-1',
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enabled: true,
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language: 'eng',
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kind: 'main',
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id: 'random'
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}, {
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label: 'audio-tracks-2',
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enabled: false,
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language: 'fr',
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kind: 'alternative',
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id: 'random'
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}, {
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label: 'audio-tracks-3',
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enabled: false,
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language: 'eng',
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kind: 'main-desc',
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id: 'random'
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}]
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```
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## Startup (how tracks are added and used)
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> AudioTrack & AudioTrackList live in video.js
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1. `HLS` creates a `MasterPlaylistController` and watches for the `loadedmetadata` event
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1. `HLS` parses the m3u8 using the `MasterPlaylistController`
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1. `MasterPlaylistController` creates a `PlaylistLoader` for the master m3u8
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1. `MasterPlaylistController` creates `PlaylistLoader`s for every audio playlist
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1. `MasterPlaylistController` creates a `SegmentLoader` for the main m3u8
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1. `MasterPlaylistController` creates a `SegmentLoader` for a potential audio playlist
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1. `HLS` sees the `loadedmetadata` and finds the currently selected MediaGroup and all the metadata
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1. `HLS` removes all `AudioTrack`s from the `AudioTrackList`
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1. `HLS` created `AudioTrack`s for the MediaGroup and adds them to the `AudioTrackList`
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1. `HLS` calls `MasterPlaylistController`s `useAudio` with no arguments (causes it to use the currently enabled audio)
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1. `MasterPlaylistController` turns off the current audio `PlaylistLoader` if it is on
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1. `MasterPlaylistController` maps the `label` to the `PlaylistLoader` containing the audio
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1. `MasterPlaylistController` turns on that `PlaylistLoader` and the Corresponding `SegmentLoader` (master or audio only)
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1. `MediaSource`/`mux.js` determine how to mux
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## How tracks are switched
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> AudioTrack & AudioTrackList live in video.js
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1. `HLS` is setup to watch for the `changed` event on the `AudioTrackList`
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1. User selects a new `AudioTrack` from a menu (where only one track can be enabled)
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1. `AudioTrackList` enables the new `Audiotrack` and disables all others
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1. `AudioTrackList` triggers a `changed` event
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1. `HLS` sees the `changed` event and finds the newly enabled `AudioTrack`
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1. `HLS` sends the `label` for the new `AudioTrack` to `MasterPlaylistController`s `useAudio` function
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1. `MasterPlaylistController` turns off the current audio `PlaylistLoader` if it is on
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1. `MasterPlaylistController` maps the `label` to the `PlaylistLoader` containing the audio
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1. `MasterPlaylistController` maps the `label` to the `PlaylistLoader` containing the audio
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1. `MasterPlaylistController` turns on that `PlaylistLoader` and the Corresponding `SegmentLoader` (master or audio only)
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1. `MediaSource`/`mux.js` determine how to mux
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