Synology, video buffering

Plex Media Server NAS & Devices
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Server Version#: 1.18.8.2527
Player Version#: LG Plex 4.29.0

Hello, I have been using PLEX for a long time. This is the first time my video is buffering when I add subtitles. I tried 4 different subtitles, whenever I do so, the CPU jumps from 8% to 99%. What seems to be the problem? I do not wish to download the file again, as it is more than 9 series of TWD. Other shows, videos are working fine with subtitles. Thank you for your reply.

I have set to burn subtitles in the the picture, but that did not help and all other videos started buffering too. I also tried lowering the quality from 1080p to 720p, did not help either.

The video is 307MB, 1 hour long.

My file looks like this: plex>shows>The Walking Dead>Season 3>E01>TWDS03E01.mkv, subtitles.english.avi

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Which client type is playing?
Which file format do the subtitles have?

(Don’t use a subfolder per episode. Don’t use abbreviations.)

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Client LG Plex 4.29.0.
Subtitles are .srt UTF-8.
I am not using abbreviations, that was just to show an example-

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I think the issue is the Avi file format. Which video codec is used inside?

That’s an abbreviation TWDS03E01.mkv

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Well as I said, it was only an example.
The Walking Dead - 03x03 - Walk with Me.mkv
The Walking Dead S03E03.czech.srt

  • Codec HEVC

  • BitovĂ˝ tok 952 kbps (bit rate)

  • Bit Depth 8

  • Chroma Subsampling 4:2:0

  • Coded Height 1080

  • Coded Width 1920

  • Color Range tv

  • Frame Rate 23.976 fps

  • Height 1080

  • Úroveň 4.0 (level)

  • Profil main

  • Ref Frames 1

  • Width 1920

  • Display Title 1080p (HEVC Main)

  • Codec OPUS

  • KanĂĄly 6 (channels)

  • BitovĂ˝ tok 138 kbps (bit rate)

  • Jazyk English

  • Audio Channel Layout 5.1

  • Sampling Rate 48000 Hz

  • Display Title English (OPUS 5.1)

  • Codec SRT

  • Jazyk English

  • Display Title English (SRT)

  • Codec SRT

  • Jazyk Česky (language Czech)

  • Format SRT

  • Display Title Česky (SRT External)

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That won’t work either.
If Plex shall use these subtitles, they must be named exactly like the video file.
And the video file must use the s00e00 season/episode code.

The Walking Dead - s03e03 - Walk with Me.mkv
The Walking Dead - s03e03 - Walk with Me.cze.srt

Are you 100% positive that your TV is able to play videos in HEVC video codec?

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To add to Otto,

Which Synology model?
Most Synology models cannot burn subtitles.
Only the DSxx18+ and newer Intel CPUs can transcode HEVC to H.264.

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I transformed 1 video to H.264 using external software. That helped.
DS218play.
Is there a way my NAS or plex could transform all other videos?

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No. HEVC is particularly hard to transcode. Without dedicated transcoder hardware, there is no way to do this in real-time.
(Keep in mind that Plex needs a Plex Pass subscription to be able to use hardware transcoders.)

You either need a much-stronger server or use only client devices which can handle the file as-is.

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Check to see if the audio is transcoding. If it is, enabling any subtitle type forces a video transcode.

With the Plex LG SmartTV app:

  • Enabling image subtitles, PGS & VOBSUB, forces a video transcode.
  • If the audio is transcoding, enabling any subtitle format, including SRT, forces a video transcode.

Edit: Ran a quick test. Opus transcodes to AAC when using the Plex app (v4.29.0) on my LG.
From Tautulli:

opus
opus703×711 71.6 KB

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Both of these force a video transcode.

In the Plex app, set subtitles to either Auto or Burn Image Formats Only.

Also, leave quality at Maximum / Original.

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How do I check if the audio is transcoding?

I have two 4K (HEVC) files. One load nicely with subtitles, the other one wont even load with subtitles.

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During playback use the Now Playing section in the Plex Dashboard to see if the audio or video is transcoding.

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I see that using some kind of sublitles forces to change “Video can be streamed directly: True” - everything works fine, I change the subtitles to other, it changes to “Video can be streamed directly: False” and lags.
I dont know the difference in subtitles, they are both downloaded from the same source.
How can I force to keep the video (and audio) as “Can be streamed directly: True”?

Using the dashboard, when using the “right” subtitles, I see audio and video as “Direct Stream”. Switching to “bad” subtitles, video is being transcoded to 1080p, H.264.

SnĂ­mek obrazovky 2020-04-05 v 19.21.01
Snímek obrazovky 2020-04-05 v 19.21.01629×795 207 KB
SnĂ­mek obrazovky 2020-04-05 v 19.23.23
Snímek obrazovky 2020-04-05 v 19.23.23626×750 375 KB

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You have to align your media with the capabilities of your client(s).

For example, I have an LG B7 OLED.

  • It supports H.264 & H.265 video up to 4K resolution, including HDR10, HLG, & Dolby Vision.
  • It supports Dolby Digital, Dolby Digital Plus, dts, and AAC audio.

The LG Plex app will direct play any combination of the above video & audio streams.

However, as previously mentioned, the Plex app does not support direct play of PGS or VOBSUB subtitles. Enabling either forces a video transcode. Also, if the audio is transcoding, then any subtitle forces a video transcode.

So, when I use the Plex app with subtitles and want to avoid transcoding, I have to do the following:

  • Choose a video stream that direct plays
  • Choose an audio stream that direct plays
  • Use SRT subtitles.

Using the Walking Dead episode you provided earlier as an example:

  • Video: HEVC, 1080p, ← this should direct play
  • Audio: Opus 5.1 ← Not supported by LG TV, so will transcode.
  • Subtitles: SRT ← If enabled, forces a video transcode because audio is transcoding

So, to direct play that episode with subtitles, you need an audio track that also direct plays.

If you have access to the discs, you could re-rip and run the video through Handbrake.

If not, look at a tool such as XMedia Recode. With it you can transcode the audio to a supported format such as Dolby Digital (AC3) 5.1. You can also tell it to copy, not transcode/modify, the video & subtitle tracks. Transcoding audio is not CPU intensive, so the process should not take long (minutes, not hours).

2 Likes
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Thank you, I will definitely try that tool (but I have to find MacOS version), as I have some 4K videos with weird audio formats. (I gave up on The Walking Dead and watched it without subtitles) .

You seem to know a lot. Do you know If it is possible to change audio track as same as subtitles? E.g. I would download a movie in czech language, and If I wanted I could swap the audio to english just as I swap the subtitles. (I have some movies with different languages, EN and DE, but I would also like to add CZ). Does it work as same as with subtitles, just insert it into the folder?

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With the Plex LG app, PGS subtitles force a video transcode.

That is why Godzilla is transcoding to 1080p.

It should direct play with SRT subtitles, because both the video & audio direct play when subtitles are disabled.

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Yeah, It will work for this file only. Some other files dont work either with SRT or PGS

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No. For audio you have to mux it into the MKV/MP4 file.

MKVToolNix is a good tool for remuxing MKV files. You can add/delete/re-order video/audio/subtitle tracks. It also has a nice header editor and chapter editor. Download from Fosshub.

For MP4 files, look at Subler. It started as a way to add subtitles to MP4 files, but now can mux in audio & video tracks and also search for metadata.

Subler can transcode AC3 audio to AAC. I’ve never tried it with Opus, so not sure if it will be of any help.

You can also use Handbrake. It requires more steps, but it works. Basically, you set the video to transcode as fast as possible. It will look horrible, but you don’t care. Then choose the desired audio format. The file will process rather quickly. When finished, use MKVToolNix or Subler to import the audio track into the original file.

Using Walking Dead as an example:

Load Walking Dead episode into Handbrake.
Pick a preset that will run fast, such as VeryFast 1080p30.
On Summary tab, set Format to MKV.
On the video tab, set the FPS to match the original file (23.976 for this example)
Set the Encoder Preset to UltraFast (all way to left)
Set the Constant Quality slider to 51 (all way to left)
On the audio tab set your desired audio output (AC3 5.1, etc).
Delete any subtitle sections.

Process the video. The output will look horrible, but will have the desired audio formats.

Load original file into MKVToolNix Multiplexor.
Drag Handbrake output into main window.
You should get a popup window with options. Select “Add as new source files…”
You’ll see all the video/audio/subtitle tracks for both files listed.
Select the ones you want to keep. You can re-order the tracks by dragging them up/down. Suggest you make the new audio track the first audio track in the file, so Plex will choose it by default.
Click “Start Multiplexing” to process things.

You can use the Header and Chapter editors if desired.

Load final output into Plex and enjoy :slight_smile:

The process is rather laborious, but it does work. I use it as a last resort when I need to process and audio track and have no other option.

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