You’re running into the limitations of the Plex client for LG TVs and the LG TV itself.
Plex app for LG TV
- Enabling image based subtitles, PGS & VOBSUB, results in a video transcode.
- If the audio is transcoding, enabling any subtitle results in a video transcode.
- If the audio is not transcoding, enabling SRT or VTT subtitles will not cause a video transcode.
- SSA/ASS Subtitles: If Burn Subtitles = Automatic, enabling SSA/ASS subtitles results in a video transcode. If Burn Subtitles = Image Formats Only and audio is not transcoding, SSA/ASS subtitles will not result in a video transcode, but some formatting may be lost.
LG TVs
- 2020 & newer LG TVs do not support TrueHD or dts audio formats. If you play media with one of those audio formats it will be transcoded by the Plex Media Server to a supported format (EAC3, AC3, etc).
- 2019 & older sets support dts and dts-HD, but not TrueHD.
Plex Media Server
- Transcoded video is converted to 8-bit H.264, which does not support HDR.
- If TrueHD or Dolby Digital Plus audio is transcoded, any accompanying Atmos information is lost.
Therefore, when playing HDR media using the Plex LG app:
- Do not enable image based subtitles.
- If possible, avoid TrueHD & dts audio. If they cannot be avoided, do not enable subtitles.
Note:
- Movies on Blu-ray discs with TrueHD audio also include a Dolby Digital (AC-3) version of the audio track.
- Movies with dts-HD audio also have a dts version of the audio track. They do not have to include a Dolby Digital version.
