Server Version#: 1.32.7.7621
Player Version#: 5.71.1 (platform: 6.3.3)
The player on my LG TV unfortunately does not support TrueHD audio playback. It makes sense to me for Plex transcoding the audio to something that is supported (EAC3 in my case). However, Plex does not only transcode the audio, it also transcodes the video in this case.
When I then select the AC3 audio track that is included in the video file, both video and audio are direct play. When I disable the subtitle (srt in this case), but leave TrueHD on, the video is direct play and only the audio is transcoded.
This behavior is not what I expected. The hardware I run Plex on does not have hardware encoding available, so transcoding 4K video makes playback impossible due to all the interruptions. How can I make sure only the audio gets transcoded with subtitles on and the video remains direct play? For this specific playback it is not an issue because I can select a different audio track with the right codec, but sometimes TrueHD is all that is included in which case the video becomes unplayable for me.
You are running into the limitations of the Plex LG app and the limitations of your server hardware.
The solution is:
Use audio formats that do not require transcoding (no TrueHD and, depending on model, no DTS).
Use a Plex client that can direct stream subtitles when the audio or video is transcoding (ex: Android, Android TV devices).
Additional Info:
As you’ve discovered, the TV does not support TrueHD audio. It cannot decode and play it through the TV speakers or pass it to other audio equipment via HDMI-eARC. The tech specs at lg.com indicate that DTS audio formats are also not supported.
When an unsupported audio format is selected, Plex will transcode it to a supported format.
With the Plex LG app, when the media is direct streaming, such as when audio is transcoding, enabling any type of subtitle results in a video transcode. The subtitles are burned into the video, becoming part of the video stream.
Plex must do this due to a limitation in the streaming protocol used between the TV and the server. If Plex sent them as a separate track, they would become out of sync with the audio and video tracks.
Note that subtitle burning uses the CPU, even if the video is transcoded using hardware acceleration. The burning process is single threaded, unable to take advantage of multiple cores.
As a result, lower power CPUs and those with slower cores will struggle to burn subtitles.
CPUs such as the Celeron J3xxx will struggle to burn subtitles into 1080p media. Recent Intel Core CPUs can burn subtitles into 1080p media. I’m not sure if any CPU can burn subtitles into 4K media, given the number of pixels that must be processed for each video frame.
Thank you for your very detailed response, I understand now why this happens.
Maybe I will invest in a Nvidia Shield at some point. For now, I installed “tdarr” to ensure there always exists an EAC3 sound track which is to be created (if it does not exists) from the highest codec quality available.
Tdarr works like a charm after some initial confusion about the set-up. My media is now automatically checked by tdarr and if needed, the EAC3 track gets added which takes less than 10 mins for a 2 hour video.
This is true for the plex app on LG TV’s. However, i noticed that using the jellyfin app it is supported (at least with dolby truehd)
For example:
I have the matrix resurrection in 4k HDR, with an AC5.1 track, an DTS X track and trueHD with atmos track.
Also i have dutch external subtitles.
When in the plex app i play the movie with trueHD i get a 4k HDR direct stream with EAC7.1 audio. Nice. However, when i enable subs it will start transcoding to 4k SDR and will try to burn subtitles. (And thus, melting the server).
When i do this exact same scenario in jellyfin on LG i get direct stream 4K hdr, 6.1 DD audio, and direct stream subtitles.
So, i am unsure why it does work in there.
Edit: i figured it out, Jellyfin does some logic when it detects it can’t play trueHD, it will select the next best audio track it can play, so it falls back to the AC5.1 track. Maybe this is some logic that can be implemented in plex?
This logic would be great, not only for LG, but with other clients as well.
Other capability would be capability for Plex to show srt subtitles with 4K HDR movies without server transcoding. With Jellyfin this is already possible with LG TV.
