Does the Plex Server stream all audio tracks to the Plex Client at once?

General Discussions
1

I have a movie on my Plex Server encoded with HEVC and with two audio tracks:

Size of HEVC stream: 1.03 GB
Size of TrueHD stream: 2.49 GB
Size of AC3 stream: 0.40 GB

Total size of all streams: 3.93 GB (and this is the size of the .MKV file on the hard drive).

If my Plex Client is playing this movie with the TrueHD audio track then I would need a bandwidth of about 5 mbps (megabits per second).

On the other hand, with the AC3 audio, I would need a bandwidth of about 2.17 mbps, which is less than half of the first case.

Is the Plex Server shoving the TrueHD audio stream down the wire even when the Plex Client has requested the (much) smaller AC3 stream?

2

That depends on the playback mode.
https://support.plex.tv/articles/200250387-streaming-media-direct-play-and-direct-stream/

Direct Play is indeed transferring the whole file as-is.
Direct Stream on the other hand will only transfer the selected streams.

3

I find that article very confusing. It states:

Direct Play

Media can be Direct Played on a device if the source media is compatible with the client’s playback capabilities, meaning that the source media:

  • Is stored in a compatible file container
  • Is encoded in a compatible bitrate
  • Is encoded with compatible codecs
  • Is a compatible resolution

Basically, the file is 100% compatible with your device.

If the file really is 100% compatible, then so are the contained streams. So why would the Server send a stream it should know the client is not asking for?

If the client is running on an AppleTV 4K, how do I force the Server to send only the video stream and the audio stream the Client is actually playing? This is relevant as an unselected (by the Client) DTS-HD 7.1 audio stream can be more than twice the size of the HEVC video stream.

Every movie I have is in an MKV container, HEVC video and various audio tracks. Every movie plays Direct Play. I have never seen it say Direct Stream.

4

Your combination of a lossless audio track and an extremely compressed video track is rather unusual methinks.

You could observe it e.g. when trying to play MKV in the browser (with compatible video and audio of course).

5

That would no longer be direct play. In order to remove the undesired streams, Plex would have to re-mux the file in real-time (direct stream).

Disable direct play in the Plex app’s settings on the Apple TV 4K. This will force the server to direct stream if there are compatible streams contained within the file (which presumably there are). This won’t cause transcoding of the streams; they will just be re-muxed.

6

@pommesmatte Yes, I would agree with you. I have several movies with Dolby Atmos tracks packaged in MKV containers. These won’t play directly on an AppleTV, forcing the Server to transcode them to FLAC. So I’ve started adding a FLAC 7.1 stream to these containers, keeping the Dolby Atmos streams as well (who knows, one day maybe the AppleTV - or some other box - will stream that directly).

I’ve been playing around with the Plex Client’s Advanced Allow Direct Play settings. I’ve noticed some strange things so far. In all cases, the video is HEVC. If I tell the Client to select the FLAC audio stream, then turning off Direct Play indeed forces the Client into Direct Stream mode. Everything works great and the Client reports that the bitstream rate is about half of what it was when it was playing the same movie in Direct Play mode. This makes sense as the Dolby Atmos track is no longer being pushed down the wire.

But if I select any audio track supported by the AppleTV other than FLAC (i.e. AAC, AC3, etc.) something weird happens: the newly selected audio track plays as expected, but there is no video. The screen is black with a “buffering”-ish looking icon spinning in the middle of the screen. This never changes as the movie continues to play and audio continues to play as expected. The Client’s UI remains responsive, so I can both confirm that the Server remains in Direct Stream mode and I can go back and choose the FLAC track. When I do this, all is well as the video is now displayed, and of course, the FLAC track plays.

This feels like a bug in the Client. Both HEVC and whatever non-FLAC track I selected are compatible with the AppleTV. I don’t know what’s so special about the HEVC/FLAC combo such that this is the only combo that plays correctly.