Offline DASH `StreamKey` mismatch between `DownloadHelper` and `DashMediaSource`

I’ve noticed that there is a mismatch between the track groups advertised by DownloadHelper, and those advertised by DashMediaSource.

Consider a single-period DASH VOD stream with the following tracks, as advertised by DownloadHelper:

• Period 0
  • TrackGroup 0: video tracks (6 representations)
  • TrackGroup 1: application/cea-708 (embedded)
  • TrackGroup 2: audio/eac3 (1 representation)
  • TrackGroup 3: audio/mp4-latm (1 representation)
  • TrackGroup 4: text/vtt (1 representation)
  • TrackGroup 5: application/x-emsg (embedded)

Track groups at index 1 and 5 are embedded in the video segments.

In this example, let’s suppose we would like to download:

• the highest bitrate video track (StreamKey(0,0,1))
• the MP4 audio track (StreamKey(0,3,0))

When ExoPlayer re-parses the manifest to get the segments to download, it is unaware of the embedded CEA-708 and SCTE track groups, so the DashMediaSource looks like this:

• Period 0
  • TrackGroup 0: video tracks (6 representations)
  • TrackGroup 1: audio/eac3 (1 representation)
  • TrackGroup 2: audio/mp4-latm (1 representation)
  • TrackGroup 3: text/vtt (1 representation)

…so the copied manifest (persisting only provided StreamKeys) contains the wrong tracks, in this example, using our previously selected StreamKeys, the WebVTT track not MP4 Audio track will be downloaded, resulting in a silent playback (but at least you get subtitles I guess!).

[ojw28]

Closing this on the assumption it’s an app issue as described, but please let us know if not. Doing so here is fine as long as the issue isn’t locked (it doesn’t matter that it’s closed; we can re-open it if needed).

We updated the StreamKey documentation to try and make it a little clearer in the commit referenced above.

[benlancaster]

Is there another way to allow tracks from MappingTrackSelector.MappedTrackInfo.unmappedTrackGroups to be downloaded?

If the track ends up in unmappedTrackGroups then that implies that it’s not playable, assuming you’re using equivalent RenderersFactorys for downloading and playback, which we’d expect to be the case. So I think my question would be: Why would you want to do this 😃? I suspect that you may have a somewhat unusual use case.

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