This is how the Plex LG app behaves with subtitles:
- Enabling image based subtitles, PGS or VOBSUB, results in a video transcode.
- If audio is transcoding, enabling any subtitle results in a video transcode.
- If audio is direct playing, text subtitles, SRT & VTT, direct play.
- For ASS/SSA subtitles:
- If Burn Subtitles = Automatic in the Plex LG app, enabling ASS/SSA subtitles results in a video transcode.
- If Burn Subtitles = Only Image Formats, ASS/SSA subtitles, direct play, but formatting information (color, location, etc) may be lost.
Short Answer:
Apple TV: I do not know.
Shield: Yes.
Note: Direct playing SRT or PGS/VOBSUB subtitles while the audio is transcoding is part of the Plex Android TV client. It is not specific to the Shield. So, you do not necessarily need a Shield.
For example, an Amazon Fire Stick 4K will also direct play subtitles when the audio is transcoding. All the audio will be converted to either Dolby Digital or Dolby Digital Plus, so no dts or TrueHD (and in some instance you’ll receive 2.0 audio even if the source is 5.1). However, depending on your listening environment that may not matter to you.
Long Answer:
I do not know how the AppleTV handles subtitles (I do not have an AppleTV with which to test).
Due to Apple restrictions, the AppleTV cannot passthrough TrueHD audio. If I remember correctly, Plex Media Server transcodes TrueHD to FLAC. The AppleTV then passes it to attached audio equipment as multi-channel PCM. The TrueHD → FLAC → PCM action was reported on a thread several months ago. FLAC & PCM are both lossless, so there should be no audio degradation due to the conversion. However, any Atmos data is lost in the conversion process.
The AppleTV passes dts audio as multi-channel PCM.
A Shield will direct play subtitles even if the audio and/or video is transcoding. This was added in v8.0 of the Plex Android TV client.
So basically, you could play a 4K HDR video, have the dts audio transcoding, enable PGS or SRT subtitles, and the video won’t transcode.
A Shield will passthrough TrueHD (including Atmos) and dts/-HD/:X audio if the attached equipment supports those formats. If they are not supported, then the audio will be transcoded to a supported format.
Note that the LG CX will not passthrough dts audio formats from equipment attached to one of the TV’s HDMI inputs. When LG removed dts audio support from their TVs they also blocked dts passthrough from attached devices.
Also, passing TrueHD and dts-HD through the TV requires that both the TV and the attached audio equipment support HDMI-eARC. HDMI-ARC does not have the bandwidth to pass TrueHD or dts-HD. HDMI-ARC is limited to Dolby Digital, Dolby Digital Plus, & dts at 5.1 channels or less. PCM 2.0 is also supported.
If possible, it is best to connect the Shield (or any streaming device) directly to the audio equipment. This lets the most audio formats direct play.
Example: Streaming device <–HDMI–> Receiver/Soundbar<–HDMI–> LG CX
It is not always possible, due to a lack of HDMI inputs on the audio equipment, or for other reasons. You can still connect the streaming device directly to the LG, but dts audio will be transcoded due to the limitations of the TV.
Example: Streaming device <–HDMI–> LG CX <–HDMI-eARC → Audio Equipment
I’ve kind of rambled a bit. Hopefully I answered your question. If I have not, or worse, left you horribly confused, let me know and I’ll try to straighten things out.