See this thread: [INFO] Plex, 4k, transcoding, and you - aka the rules of 4k
You are running into the limitations of the Plex SmartTV app on Samsung and LG (and maybe other) TVs.
- Enabling PGS or VOBSUB subtitles results in a video transcode.
- If the audio is transcoding, enabling SRT (text) subtitles results in a video transcode.
There are also limitations of the TV itself.
- The TV does not support TrueHD or dts audio, so playing either via the Plex app results in a audio transcode.
- When using TV apps, neither Samsung or LG support passing TrueHD audio over eARC, therefore, it will be transcoded.
To use the Plex app on the TV when watching HDR media:
- Never use image based subtitles, PGS & VOBSUB.
- To use text (SRT) subtitles, choose audio formats that do not transcode: AC3, EAC3, & AAC.
Regarding a 2019 Nvidia Shield Pro with the Plex Android TV app:
- HDR10 media direct plays.
- Dolby Vision media is a mixed bag, as only some DV formats are supported. Unsupported DV formats transcode.
- The Plex AndroidTV app direct plays subtitles even if the audio is transcoding, so no video transcoding if you enable PGS/VOBSUB/SRT subtitles. Supported since v8.0.0.
With Shield → Samsung → Sonos
- The Samsung should pass TrueHD + Atmos to the Sonos via HDMI-eARC (both devices must support eARC).
- I do not know if it will pass dts audio formats (LG blocks dts passthrough in their current models for some reason).
