Help with buffering issue mystery

Plex Media Server
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Server Version#: 1.24.1.4931
Player Version#: ?? (Roku Player, Samsung Player)

So here is my issue, I cant seem to get my movies to not buffer whenever I add in a subtitle track. I have a synology 918+ (I have plexpass and have enabled HW transcoding). I am only running one stream at a time in 1080p and not matter what I continuously get buffering (every min or two) issues and even “the server is not powerful enough to convert this video for smooth playback” messages. All videos are H.264 encoded and will direct stream with no subtitles. My cpu only reads at 30% when transcoding and I dont understand how or why I cannot even run a single stream without buffering.

One item I do find curious is watching the bandwidth monitor when transcoding it fluctuates wildly. 32mpbs then 16kbps and so on back and forth - it is a a lot more steady during the direct stream plays.

Any help would be greatly appreciated.

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What’s your audio codec?
If you’re transcoding audio, enabling subtitles will automatically cause a video transcoding too.

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Usually whatever audio source was on the original disc. I don’t tend to process them unless it was a VLC encode originally. Does having both an audio and video transcode slow it down a great deal - to cause more buffering? And is there a way to play pgs subtitles without transcoding - I was under the impression I was always going to be transcoding when using subtitles. Thanks.

I just forced direct play on my roku tv and it played flawlessly without subtitles - both audio and video no transcode - but once I add the subtitles it starts buffering endlessly.

I am just wanting to figure out a way to get them to play with subtitles if needed as I have a family member who needs them sometimes. Would lowering the bitrate its transcoded at help or any other setting?

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I believe only Nvidia Shield supports PGS, and any other player will request the transcoding of it. Another thing, subtitles are always transcoded in CPU not in GPU.
Try using srt subs (don’t even try to convert your PGS to srt, it’s straight garbage)

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You are most likely running into a) the limited subtitle support on various Plex clients and b) running Plex on a Linux based system with a low power CPU.

Subtitle support varies widely across Plex clients.

For example, the Plex LG webOS app does not support direct playing image based subtitles such as PGS or VOBSUB. When such a subtitle track is enabled, Plex must transcode the video and add (burn) the subtitles into the video stream.

On Linux systems with Intel CPUs such as a DS918+, adding the subtitles to the video stream occurs on the CPU, even if the video transcode uses hardware acceleration. This is due to Intel video driver limitations.

Additionally, burning subtitles is a single threaded process, so it cannot take advantage of multi-core CPUs.

Now add a low power CPU such as the Celeron J3455 in the DS918+ to the mix.

Basically, the Celeron CPU is not powerful enough to burn subtitles into the video stream in real time.

As a result, the end-user experiences buffering, waiting on the server to ship it more video with subtitles.

The overall CPU utilization appears low because the process is single threaded. One of the four cores in the Celeron J3455 will be near 100%, but the others are most likely much lower, handling audio transcoding or other tasks.

One way around the problem is to find a combination of video, audio, and subtitle formats that do not result in subtitle burning.

Unfortunately, there is no “master list” of Plex clients and how they handle subtitles.

I do not know how the Roku handles subtitles.

I believe the Plex Samsung (Tizen) app behaves similarly to the LG webOS app:

  • Enabling PGS or VOBSUB subtitles results in a video transcode & subtitle burning.
  • If the audio is transcoding, enabling any subtitle format results in a video transcode and subtitle burning.
  • If the audio is direct playing, text subtitles (SRT & VTT) direct play.
  • Enabling SSA/ASS subtitles results in video transcoding and subtitle burning. There is a workaround, but formatting information such as color, font, etc is lost.

Other Plex clients handle subtitles better. For example, the Plex Android TV app will direct play both image and text based subtitles even if the video and/or audio is transcoding (so no subtitle burning). Unfortunately, enabling SSA/ASS subtitles result in video transcoding & subtitle burning.

As mentioned above, one way around the problem is to find a combination of video, audio, and subtitles that does not involve burning subtitles into the video stream.

In general, this will mean using Dolby Digital (AC3), Dolby Digital Plus (EAC3), or AAC audio and SRT (text) subtitles.

An alternative solution is to use Handbrake or similar tools to burn the subtitles into the video stream before adding the media to your server. The downside is the time involved to process the video and also that the subtitles cannot be disabled since they are now part of the video stream.

You could also run Plex Media Server on a separate system, with a CPU powerful enough to support subtitle burning (ex: Intel NUC with an i5 CPU). The media would remain on the NAS.

1 Like
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The NUC route looks to be my best bet it seems to allow all my TVs to access. What specs would you recommend to accomplish the ability to transcode 1080p with subtitles on the fly? Again one or two streams only typically and all local area network. I want to be sure what I get can handle it but don’t want to go overboard when I could use the extra to get more movies :slight_smile:

Thanks again.

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This answer should be the first one to come up in the results when someone search for the words “subtitle transcoding” on this forum.

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I run Plex on a DS918+. Instead of going with a separate system, I use clients that minimize transcoding and subtitle burning. I use a Shield as my client. I’ve 3-4 friends that stream remotely and they use Amazon Firesticks and an Apple TV.

My setup: Shield <–HDMI–> Denon receiver <–HDMI–> LG TV

Using a streaming stick/box/etc might be an alternative for you instead of running Plex on another system.

Regarding the NUC, there’s some general comments below. I hesitate to get too specific, as I still run PMS on the Synology. I’m sure others will add their opinion and can help if you’ve specific questions.


Low Cost Android TV Client: Amazon FireStick 4K, ~$50 USD, frequently on sale for less.

Pros:

  • Cost
  • Direct plays PGS/VOBSUB/SRT subtitles, even if audio and/or video is transcoding (no burning).
  • Direct plays 4K HDR10 movies and some Dolby Vision formats.
  • Supports Dolby Digital Plus + Atmos, the format used by streaming services such as Netflix, Amazon, etc.

Cons:

  • SSA/ASS subtitles require a video transcode and subtitle burning (applies to all Plex Android clients).
  • No TrueHD passthrough. TrueHD, including TrueHD + Atmos (found on Blu-ray discs), will be transcoded to Dolby Digital or Dolby Digital Plus by the Plex Media Server. Atmos information, if present, is lost in the process.
  • No dts passthrough. All dts audio formats are converted by the FireStick to Dolby Digital or Dolby Digital Plus. There is no transcoding load on the Plex Server. However, only the core dts 5.1 audio is processed. The “-HD MA” or “:X” information is dropped.
  • No Google Play store access. Amazon app store only.

Comments:

A good alternative to the Plex Samsung and LG TV apps, especially if you only need 5.1 audio or are using the TV’s speakers.

Not everyone likes the Amazon user interface. However, once you’re inside the Plex app, it looks like the Plex app on any other Android TV device.

The Amazon app store publishes updates less frequently than Google. As a result, the Plex app on FireTV devices is frequently a few versions behind the Google Play store version. This is not necessarily bad, just something of which to be aware.

The FireTV Cube v2, while more expensive (it has a built in Echo), is also an option. Be wary of older and non-4K capable FireTV devices. They may have a lower power CPU or less memory and difficulty handling some Plex streams (Translation: I’ve the 4K stick and it works fine. I’ve had a lot of trouble with older FireTV sticks/boxes/cubes). Tech Info for Amazon Fire devices: Fire TV Device Specifications

There are a lot of other Android TV boxes on the market that will have similar capabilities. Do you research before buying one. Many of the “no-name” models will never receive Android updates, the manufacturer may not support the desired audio codecs (check for DDP+Atmos), etc. If a certain make/model looks promising, search the Plex forum to see if others have used it and if it worked well for them.


High Cost Android TV Client: Nvidia Shield Pro 2019, ~$200 USD.

Pros:

  • Almost everything direct plays when using a Shield.
  • Supports 4K HDR10 and some Dolby Vision formats.
  • Supports passthrough of any audio format you’ll find on a commercial disc, including TrueHD, TrueHD + Atmos, dts-HD MA, & dts:X.
  • Direct plays PGS/VOBSUB/SRT subtitles even if video/audio is transcoding.
  • Google Play Store

Cons:

  • Cost
  • SSA/ASS subtitles require a video transcode and subtitle burning (applies to all Plex Android clients).

Comments:

One of the best Plex clients for a home theater setup, especially if you’re using a higher-end soundbar or AV receiver with TrueHD+Atmos/dts:X support.

I’ve the 2015 Shield Pro. As long as I avoid Dolby Vision video and SSA/ASS subtitles, it direct plays everything I throw at it (The 2019 models added support for Dolby Vision).

Note that the non-Pro “tube” model has problems with high bit rate media such as 4K HDR movies with Atmos or dts:X audio. It is prone to audio dropouts and stuttering. It is most likely OK for 1080p and lower video, but if you plan to play 4K media go with the Pro model.


Plex Media Server System

My comments in this area will be less specific, since, as mentioned above, I still run PMS on my DS918+.

Go with an Intel 8th gen or higher CPU. Specifically, one with UHD 630 graphics or better. Intel improves the Quick Sync Video GPU in their systems with each generation. From comments on the forum, UHD 630 has better video quality than prior generations. It can also decode 4K HEVC 10-bit video if needed.

Do your homework before buying a system with an Intel 11th Gen+ CPU with Xe graphics (i5 & i7 CPUs).

  • Linux: Intel released buggy Linux video drivers for systems with Xe graphics. This is causing transcoding headaches for users running Linux on such systems, especially when transcoding 4K HDR media. Plex developers are working on support, but as of today, it is problematic. There are several threads in the forum on the topic. Here is one: Anyone have been able to HW transcode on an Intel NUC 11 (Iris Xe)? .
  • Windows: I don’t know how well Xe graphics are supported on Windows. You can search the server-windows section of the forum. I saw several posts, but nothing definitive (I admittedly did not perform an exhaustive search).

So, what CPU? At least one with equivalent performance to an i3-8100. I’m basing that on forum posts where people have had success with that CPU (search for “i3-8100”). Most NUCs/Small Form Factor computers will have mobile or embedded CPUs, not the desktop version. You can compare Passmark scores to see how the CPUs stack up against each other. Remember to look at the single thread scores, not just the overall scores, since subtitle burning is single threaded.

An i5 would give you growth room, which could come in handy if you support additional users. Many times remote streaming requires transcoding audio due to bandwidth constraints. Audio transcoding is not CPU intensive, but it will add to the overall load of the system. Also, some Plex background tasks will run faster. For example, the new Sonic Analysis for music uses the cpu, not the gpu, and is compute intensive. The faster the CPU, the faster the analysis is performed.

Some Passmark scores:
Celeron J3455: Single thread rating: 811
i3-8100: Single thread rating: 2243

Make sure the SSD or hard drive has enough room to hold your Plex data folder (and OS, temp file space, etc). Look at your current Plex data folder size and leave room for growth.

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