Playing 5.1 PCM audio with tunneling mode.

[navluis]

[REQUIRED] Use case description

Some audio streams contain 5.1 channel-masks. When those audio streams are decoded (instead of played as passthrough) and tunneling is enabled, ExoPlayer is unable to play audio because it is using the version 1 of the av-sync header.

Proposed solution

The only thing that is required for that feature to work is to enable the lines corresponding to this comment :
// TODO: Uncomment this when [Internal ref: b/33627517] is clarified or fixed.
in file:
"library/core/src/main/java/com/google/android/exoplayer2/audio/DefaultAudioSink.java"

The reason why enabling that code will fix the issue is below:

The tunneling audio header version 1 has a fixed header-size of 16 bytes. The AudioTrack allows to write data only in multiples of the audio track frame size and because ExoPlayer attempts to fully write the header before writing the payload, it is therefore mandatory that the header size be a multiple of the frame-size. For PCM16 (2 channels) this is not a problem because the frame-size is 4, but for other channel-mask such as 5.1 (frame-size 12) ExoPlayer will be unable to fully write the audio header and therefore unable to play any audio.

Android introduced earlier a header version 2, which contains an 'offset' field which allows the frame-header to have a variable size. ExoPlayer writes the audio header itself (version 1) but Android internally is able to write the header version 2 and also sets the header size to a multiple of the frame size. This change simply delegates the task of writing the audio header to the Android framework.

Alternatives considered

n/a

[tonihei]

@andrewlewis Can you take a look at this issue?

[ojw28]

It looks like we can do the TODO for Oreo and above (SDK_INT >= 26). Note however that the V2 header was only implemented at some point after that, however (possibly only Pie and above).

[navluis]

Thanks for the prompt response to this. Will give it a try!