Subtitles even in text formats (SRT, ASS, etc.) causes transcode of HDR files

Plex Players Smart TVs
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Server Version#: 1.21.1.3830-6c22540d5
Player Version#: 5.6.1

I have a 4K HDR video using TrueHD 7.1 sound which plays in HDR mode in Plex on my LG 55UJ635V using webOS. However, turning on SRT subtitles causes transcoding of the video which also loses HDR mode. I think the issue might be that the audio is transcoded (I’m still hunting down the specs to see if this TV should support TrueHD 7.1 sound) but I don’t see why this would also trigger transcoding of video as long as the subtitles are in SRT format.

There seems to be some development of the webOS client recently, with the latest Early Access released in November. Are you aware of issues related to this and do you think it will be fixed in the near future?

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With the Plex WebOS app:

  • Enabling image based (PGS/VOBSUB) subtitles forces a video transcode.
  • If the audio is transcoding, enabling any subtitle forces a video transcode.

If the audio is not transcoding:

  • SRT/VTT subtitles direct play.
  • ASS subtitles force a video transcode if Subtitles = Automatic in app settings.
  • ASS subtitles direct play if Subtitles = Only Image Formats in app settings.

Your TV does not support TrueHD audio (specs). If you play a TrueHD audio track using the Plex app, Plex Media Server will transcode the audio to a supported format.

Movies released on Blu-ray with TrueHD audio also include a Dolby Digital (AC3) version of the soundtrack. If the AC3 audio is available in your movie file, play it instead.

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But how are subtitles related to audio? Why would transcoded audio affect subtitles in any way?

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Plex Media Server has to transcode the video to keep the video, audio, and subtitles in sync with each other.

I don’t know if it is a limitation of TV hardware, WebOS, transport protocols, etc. Someone from Plex will have to provide the details.

Edit: It is a limitation of the HLS protocol. See the following thread. The main topic is unrelated (and resolved), but the post provides info regarding LG, transcoded audio, and subtitles.

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On what do you base this assumption?
There should be no need to transcode everything to keep it all in sync.
The fact that it works without forcing a full resync on other devices also makes me doubt this is the case here…

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That the people who work for or are otherwise affiliated with Plex know how this stuff works. Also, that I am correctly interpreting what they write.

Look at the post I referenced. Even if I’m incorrect about the keeping in sync part, it still holds that LG + HLS does not support transcoded audio + subtitles.

Personal experience bears this out. I’ve had an LG TV for 3+ years. It has always behaved as I described above - if the audio is transcoding and you enable subtitles, the video will transcode. It applies to any video, not just HDR.

I would love to be wrong about this. How subtitles are handled is one of the reasons I rarely use the Plex app on my LG.

What happens on other platforms is not necessarily relevant to what happens on webOS platforms. Different platforms use different underlying video players which have different capabilities and limitations.

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The transcoding is handled by another device (nvidia shield tv).
So the stream the TV itself receives is just like any other stream.

The TV in my case is a Samsung (Q60T).
My brother has an LG OLED (which i configured) and there it works fine… no such problem.

Also, can you pls refer to where plex staff said that everything has to be transcoded in order to keep things in sync?

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The problem is explained in the post I previously linked.

  • Transcoded audio means Plex direct streams the media.
  • Plex streams to LG TVs using the HLS protocol.
  • The HLS protocol does not support subtitles when direct streaming.
  • As a result, Plex has to burn in the subtitles which requires a video transcode.

Edit: See this post for information on subtitles and Samsung TVs running Tizen.

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