I’m not sure it would help.
There are two problems:
- Celeron CPU low performance, especially single-cores.
- Limited ASS support in the Plex Android / Android TV client.
I transcoded a section of the video using Handbrake to HEVC 10-bit, Main10@L4.0@High and loaded onto my system (PMS on DS918+, Nvidia Shield Pro as client).
With no subtitles, both the L6.2 and L4.0 videos direct play, without buffering, using the Shield as a client. Both have similar bitrates, 6.2 = 1741 kbps, 4.0 = 1732 kbps.
With hardware acceleration enabled and Burn Subtitles = Automatic, both the L6.2 and L4.0 videos buffered. The J3455 Celeron cannot burn the subtitles fast enough.
With hardware acceleration disabled and Burn Subtitles = Automatic, both videos buffered. Not nearly as much as with h/w acceleration enabled, but they still buffered at some point.
The J4125 in the DS920+ is a stronger CPU than the J3455, so that is probably why you can transcode the video in software without buffering. However, you will still run into buffering at some point, when transcoding higher bitrate videos.
So even if Plex had some sort of look up table to compare bitrate vs video level and auto-switch between hardware accelerated & software transcoding, I’m not sure it would help much. At some point you’ll still hit the wall due to Celeron’s low single core capability (as mentioned above, subtitle burning is a single core process on Linux systems).
A workaround is to set Burn Subtitles = Image Formats Only in the Plex client. The video direct plays, but style information is lost. To give you an example, 23 seconds into the video, the word “Assassination” does not appear on one of the panels of paper. It does appear with Burn Subtitles = Automatic.
As for a non-workaround solution:
- Run Plex Media Server on a more powerful system, at least when burning subtitles is required.
For example, you could run PMS on a laptop or desktop Windows system with a stronger CPU. The media would still reside on the NAS.
- Look for a Plex client that better handles ASS subtitles.
The problem with the Android client is that Exoplayer, the underlying video player, has very limited support for ASS subtitles. There is ongoing work to improve things (GitHub), but it will take some time to make it into production Android releases (ExoPlayer is part of the Android OS).
Other Plex clients may handle ASS subtitles better. Plex for Windows direct plays ASS subtitles. It has mpv as an underlying video player. I believe both the iPhone and AppleTV clients also use mpv. If so, they may be able to direct play ASS subtitles (I’ve neither with which to test).
Not the news you want to read, but that is where things sit today, as best as I can tell, anyway.
