Transcode speed randomly drops to 0.6-0.8, “The server was not powerful enough to convert the video”

Plex Media Server NAS & Devices
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Server Version#: 1.21.1.3876
Player Version#: 6.7.4.6764-abf6bf3df-Plex

Hello everyone,

I’m using a Synology DS220+ to stream to my Roku Stick+ locally. I’m playing small videos (720p) successfully for a few hours, but sometimes, seemingly randomly, the video will start to pause frequently, before ultimately crashing with the message, “The server was not powerful enough to convert the video.”

Here is some info on the stream itself:

Playback:
Quality: 1.9 Mbps 720p (Original)
Direct Play (Auto)
Allow Direct Stream: Yes
Roku Captions Mode: Off
Burn Subtitles: Automatic

Streams:
Subtitles: English Forced (ASS)
Audio: English (AC3 Stereo)
Video: 720p (H.264)

Stream Info:
Playback: transcode
Format: hls
Video: H.264 • convert
Audio: English (AAC Stereo) • convert
Subtitle: burn
Speed: 0.8

When the stream is working fine, the Transcode Speed is 1.2+, and the Playback is listed as “transcode (throttled),” indicating the server is fully capable. I don’t know why the server randomly dips in transcode speed.

I am the only one using the server. The diskstation is not doing any other activities (no Docker, virtual machines, scheduled tasks, etc.). It is solely playing this one 720p video, and it fails to do so. It usually works for hours at a time, but when it struggles, it struggles incessantly. It’s really all-or-nothing.

I have looked at previous topics and have tried several things to troubleshoot this:

The video is being transcoded (H.264 -> H.264), I think for subtitle burning.
The audio is still being converted (AC3 -> AAC). I have no idea why. The Roku should be able to direct play AC3.
My NAS is on ethernet while my Roku is wireless, but my Roku can easily handle 20+ Mbps connections. It should be able to handle a measly 2 Mbps stream. I’m at my wit’s end here.

Any help would be greatly appreciated!

EDIT: formatting.

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If the option exists, try changing Burn Subtitles to Only Image Formats.

I don’t have a Roku. However, when I change the setting on my LG TV, ASS subtitles no longer force a video transcode.

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Have you monitored the NAS when it’s slow? Could be it’s doing something in the background; CPU/RAM/HDD limitation. Depending on what it is, a reboot is not necessarily gonna help if it just picks up where it left off.

My money is on the server side but it could also be your Roku. Maybe its cache get filled up after such a long time. Did you try force restarting Plex app or entire Roku?

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If I may add here?

None of the Synology CPUs, except for the Xeon based ones, are any good at subtitle burning. They never have been.

I see in the above posts where the settings / decisions have been “burn”. This is the cause of the slow transcode speed.

What’s happening in Plex when burning subtitles:

For each video frame:

  1. HW decodes the input video and deposits in a buffer
  2. The CPU then constructs and merges the text overlay into the video image
  3. HW encodes the resultant video frame

That process of construction and merge is CPU intensive.

On Synology, the best path is to avoid subtitle burning at all cost.

Media curation, i.e. Burning desired subtitles into the video in advance externally will always yield the best result.

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Unfortunately, changing Burn Subtitles to Only Image Formats results in no subtitles being displayed on my TV.

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I’ve used Resource Monitor in the DSM Desktop and can confirm that only Plex is using the CPU (aside from the DSM Desktop itself and small services using just 0.1%).

It might be the Roku, and it might be a time-duration issue. So far I haven’t experienced this issue at all when streaming to my iPhone. It’s hard to say for sure if it’s just a hardware difference, because I watch my Roku for hours while I only watch my iPhone for under an hour at a time. So I don’t know which factor (hardware or time) to attribute the difference to. I might need to conduct some experiments here.

EDIT: Duh. On iOS, both the video and audio are able to be Direct Played. I guess it’s just a hardware limitation of the Roku/TV. I’m pretty sure the Roku can’t natively play ASS subtitles, so it has to be transcoded in real-time. So that answers why the transcoding is happening, but not why it sometimes works perfectly fine, but other times isn’t.

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So let me get this straight. The client (Roku) can’t Direct Play ASS subtitles, so Plex has to transcode so the subtitles are burned into the video file. Since the server is a Synology with a CPU that isn’t tailored for transcoding, it’s unable to do this in real-time, resulting in buffering.

But how come for certain episodes with subtitle burning it transcodes completely fine but for others it’s way too slow? I’m watching the same show, just different episodes of about the same length.

EDIT: After some investigating, it seems most of the episodes have subtitles preemptively burned in, while just a few have external ones. I’m pretty sure the episodes giving me transcoding trouble are the few with external subs. That’s why I only experienced this problem seemingly randomly.

Thanks for your help!

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