4K and Subtitle problems. PGS won't play while SRT are completely unsynced

Plex Players Smart TVs
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Server Version#: Synology DS920+ 1.24.5.5173
Player Version#: unknown

Hello,

I usually watch my movies on my LG TV via nVidia Shield. Works perfectly. Now I share my library with my brother who uses a Samsung QE55Q70 TV and he has some issues.

1 - 4K movies are not played at all even though his TV is 4K capable. They are just buffering. On the server side I’ve turned off Enable HDR tone mapping since the movie will play when he selects 1080p resolution. I’ve also selected the option “Make my CPU hurt”. Enabling HDR tone mapping makes the movie buffering every few seconds. He is not able to receive a direct stream. During transcoding, that I understand happen since he chooses 1080p on a 4K movie (?), my DS920+ CPU is only working about 30-40%. My internet connection is 250 Mbps up and down and during this buffering very little is being uploaded. Hence neither my server CPU nor my internet upload speed seem to be the bottleneck here.

2 - Being Norwegians subtitles is appreciated on English speaking movies. The movies I’ve ripped contain a PGS Norwegian subtitle, that my LG and nVidia Shield play perfectly. On the Samsung however the movie won’t even start when a PGS subtitle is chosen. Again, neither the CPU, not the internet upload is anywhere near 100%. So why the issue?

3 - He can instead choose to download a SRT subtitle, however too many of these are completely out of sync. Sometimes as little as 5 seconds, other times it is basically impossible to figure out what the desync is. It is just completely off.

Are there solutions to any of these issues?

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You’re running into multiple issues: 1) Lack of PGS subtitle support in the Plex app; 2) Low power CPU in the NAS; 3) No HDR tonemapping in the Synology version of Plex Media Server.

Your choices are essentially: 1) Use only SRT subtitles; or 2) use a different Plex client that supports PGS subtitles (such as Android TV).

Subtitles:

In my experience, the subtitles from OpenSubtitles are of little use. As you’ve found out, there are many versions for each movie and most do not sync with the movie on your server.

An alternative is using tools such as Subtitle Edit to convert the PGS subtitles yourself. Since the source is the movie on your server, the resulting SRT subtitles will be in sync with the video. The downside is this is generally a time consuming process. Subtitle Edit may (will) pause frequently asking for assistance with words it cannot recognize, such as character names.

Note: “Forced” subtitles, such as when the characters in Avatar speak Na’vi, are usually easily converted with SubTitle Edit. The human intervention is still required, especially for items such as alien words. However, there are usually not many lines to convert, so the process goes rather quickly. For example, in Avatar, there are ~1800 lines of dialog. However, the Na’vi subtitle track contains ~100 lines, so conversion will take much less time than that for the entire audio track.

You can also manually download subtitles from other locations, load them into Subtitle Edit, correct the timing, then add them as external subtitles to your server. This can also be a time consuming process. See Fetching Internet Sourced & Using Your Own Subtitle Files, for a list of sources.

Plex Clients:

The Plex SmartTV app for Samsung (and LG) TVs do not support direct playing of PGS subtitles. When enabled, Plex Media Server must transcode the video and burn the subtitles into the video stream. The Celeron CPU in Synology NAS simply is not powerful enough to burn subtitles on a reliable basis.

Not all Plex clients have such restrictions. As you’ve found out, the Plex Android TV app can direct play PGS subtitles.

There are many Android TV devices available at different price points. The Nvidia Shield Pro is at the high end, ~$200 USD. The Amazon FireStick 4K sells for ~$50 USD. Both direct play PGS subtitles.

Plex clients on other streaming devices may also handle PGS subtitles (I have Android devices, so can’t say how others handle subtitles).

The Plex desktop apps, Plex for Windows/Mac, and Plex HTPC, also direct play PGS subtitles. Using one of those apps on a desktop/laptop connected to the TV may also be an option. Note that neither support displaying HDR video. It will be tonemapped to SDR by the application (not your server).

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Thank you FordGuy61 for that very detailed explanation!

I’ll investigate Subtitle Edit to see if it can be useful, although a time consuming process was something I was hoping to avoid.

You say that my CPU is is too low powered. I can understand that, but why does it not use 100% when transcoding? I only see about 30-40% CPU usage.

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The DS920+ uses the Celeron J4125 processor, which has four cores.

The Resource Monitor in DSM shows the total utilization for all four cores.

Subtitle burning is single threaded, meaning it uses just one core.

100% of one core shows up at 25% in Resource Monitor.

Were you to SSH into the NAS and use top or htop, you would likely see one core near 100% and the other cores each at a lower level. Add up the utilization for each core, divide by four, and you’ll end up in the 30% - 40% range.

The video transcoding is using the onboard GPU, not the CPU. GPU utilization is not reported in Resource Monitor, so it is not part of the 30% - 40%. I’m not aware of a way to view GPU utilization on a Synology NAS.

Here’s a post with details on how the Plex transcoder handles subtitle burning. Ignore the part about video drivers, it is not related to subtitle burning:

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Thank you again FordGuy61 for that well understood explanation. I now understand why the NAS only show about 30-40% CPU usage.

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