So I’ve been having this issue for a little while now but I’ve waited to test it with the latest public vesion of PMS.
I have PMS installed on a DS918+ with 16GB RAM, then playback on an Nvidia Shield TV, iPad, Android phone on the same network.
Whatever file I play without subtitles e.g. 4K, 1080p etc plays fine, even transcoding these files to different resolutions works fine.
CPU load also doesn’t go above 40-50% and memory usage is low. (I monitor the Plex process in DSM on Synology - resource manager)
Yet, when I enable substitles either on 4K or even 1080p CPU I get a notificaiton on the playback device that the server is not powrful enough and then intermittent playback, actually more pauses than play time.
Here is the strange part, CPU usage in the latter scenario is however low. The Plex transcodding process stays at around 24-28%, and total CPU usage doesn’t go above 40%.
What could be wrong? It’s like there is a cap for the transcodding process in this case.
Indeed 16GB but yes correct type and speed. However Plex performance is great, without subtitles.
As I mentioned no issue even with 4K HDR content, and transcoding this content. Actually CPU load is about 40-50% in most cases.
The issue is with enabling subtitles, then I get the intermittent playback and although Plex player says not powerful enough the Plex transcoder CPU load doesn’t go above 28%, so there seems to be a lot of headroom that PMS doesn’t use for some reason.
It seems to be one of thread speed. Encoding subtitles is a PITA to do (single threaded operation) and now add the 4K → 2K (which must occur first) prior to subtitles being added. If the machine supported HT, it might just do it. I’ve been trying to make time here and play with 4K->2K w/ subtitles but not been able to break away (you know how it is… those pesky customers and all ) I need to take some down-time this weekend (flu) and will see what I can find. I don’t use subtitles so I’ll need re-rip a disc first and keep a few tracks this time around
I try to read a 4k movie with french subtitle by using my Synology 918 with Plex 64 bit (latest version available on your site).
I have the same problem. CPU is low and the Plex transcodding process stays at around 30% and I get intermittent playback and a notification on the playback device (server is not powerful enough).
All my playback devices have the problem: LG 55B6V with Web OS 3, Xbox One S and iPhone 7.
That comes back to the software encoding of subtitles. It’s single threaded which, on a quad core CPU, will show as 25% load. Now add a few percentage points for 4K -> 2K transcoding in the hardware and remux + transmit, you’are at 30%.
That makes a lot of sense actually. It seemed to me intuitively that somehow CPU use was ‘capped’ around that mark and it’s because it only uses a single thread.
It’s called buying as much Pizza and Beer the transcoder team can eat LOL
Actually, they know about it. If I understand the discussion, how to hardware encode subtitles is what they’ve been working on.
What actually happens with subtitles is:
Two image planes exist (Base image plane, transparent plane where Text will go)
Base image is transcoded into the new base frame (trivial with the hardware)
Now the text must be encoded (drawn) into the image plane (no big deal there).
Where it’s slow is merging the two image planes with Text on top. Sounds easy but isn’t as easy as first thought (the “It’s only text” doesn’t apply because both are bitmaps at that point and have to be merged ‘one bit at a time’ )
When that’s done, Re-encoding for sending is trivial again in HW
I am glad I saw this post… I was considering buying a 418play but changed that to the 918+ because of another thread but, reading this, I just realised I would be in trouble as I need subtitles… Is there any recommendation to resolve this? Maybe some other NAS model? Thanks a lot!
I too am having a similar subtitle issue with my 216play. I recently made mp4’s from ISO’s with remuxed subtitles and these mp4’s would choke my Apple 4 TV and be unplayable on AFS with Plex. The Synology CPU would consume 96 to 98% of the CPU for 3-4 minuets drop to 80% and then back to 96 to 98% and become unresponsive. Ram usage hardly changed.
After ruling out everything else I could. I ended up burning the subs into the video. They play as expected on both the A4TV and the AFS(with slight freeze every so often, which is normally how they play). Even though I wanted to turn on and off subtitles as needed.
Surprisingly Synology DS Video would play the remuxed mp4’s with no issues.
Is there a reason why the Plex can’t take the subs while DS Video has no issue playing them?
If your subtitles are SRT/SSA/ASS type , they can be streamed. Check your player settings (settings - web - player ) Burn Subtitles should be “Automatic” or “Image formats only”
If “Always” this will force the PMS to burn subtitles. The NAS does not have the CPU power to do so. DS Video has no such distinction. It just sends them.
) I need to take some down-time this weekend (flu) and will see what I can find. I don’t use subtitles so I’ll need re-rip a disc first and keep a few tracks this time around
LOL