It varies by client. Subtitles may come into play as well.
For Plex SmartTV app on LG & Samsung:
- Enabling image based subtitles, PGS/VOBSUB, forces a video transcode.
- If the audio is transcoding, enabling SRT subtitles forces a video transcode.
- If the audio is direct playing/streaming, SRT subtitles direct play.
LG TVs direct play AAC, AC3, EAC3, dts, & dts-HD audio.
Samsung TVs direct play AAC, AC3, & EAC3 audio.
TrueHD audio transcodes on both.
With AndroidTV things just changed. Until recently, if the audio was transcoding, enabling subtitles forced a video transcode. This changed with the v8 client release.
I’ve no direct experience with a Roku. Not sure how it behaves.
During playback you can monitor status using the Plex Dashboard, Plex Dash app, or Tautulli.
It will be pretty obvious if you’re transcoding HDR material. Plex transcodes everything to H.264 SDR, so the HDR flag won’t show on your TV. Also, Plex does not tonemap HDR to SDR, so the colors will probably look washed out as well.
It is generally preferable to select tracks that direct play, as it puts less load on the server and there is less chance of an audio transcode causing a video transcode.
Movies with a TrueHD primary audio tracks also include an AC3 version of the primary audio track.
Movies with a dts-HD MA primary audio tracks also include a dts version of the primary audio track.
Atmos information is lost when transcoding, irrespective of source or destination codecs.
