Plex cannot handle transcoding with subtitles

[cybersteel8]

Hi team, this looks like some great progress towards understanding the problem divideby0 is experiencing, though I admit I skimmed past a lot of the posts to try and get the gist of what was being discussed.

I arrived at this thread because of the title of the thread is matching my current issue. Specifically, I have observed in Tautulli and first-hand the significant drop in transcoding speed when image-based subtitles are burned onto a video stream that needs to be transcoded.
My i5-11500 is able to keep up for a 1080p stream, but 4K streams transcode at about 0.7 speed when PGS or VOBSUB subtitles are burned in. Without subtitles, the transcode exceeds 10 and doesn’t buffer at all.

This doesn’t apply to HDR streams as this issue is also currently present so we can ignore that for now: Error message regarding OpenGL 2.0 Driver - #42 by cybersteel8

I would like to provide any further information to assist in the investigation on this transcoding behaviour, as this is a common use case in my household and the buffering when using image-based subtitle formats is quite annoying.

[cybersteel8]

My apologies, I have updated this new post’s tags appropriately. I didn’t notice them. Thank you!

[JuiceWSA]

You may want to add some details - like what you’re trying to play image based subs on.

I have various devices and clients, but few of them can Direct Play PGS subtitles. If I insist on playing PGS subtitles with those clients or devices, Plex has no choice but to Transcode for the Burn.

[SE56]

As Juice states, I would future suggest looking at srt , they will work if implemented correctly on all platforms I believe…

[JuiceWSA]

Well… that may sound easier than it actually is…

For Instance, right at this moment I am doing one of my least favorite things - running an OCR Session with Subtitle Edit to magically turn PGS subs into UTF-8 Text Based:

It’s a vile, painful process.
I’d rather have a Root Canal.

But - as mentioned, I have FEW things that can play PGS subs… so my options are limited… if I want Direct Play…lol

[Divideby0]

Yeah Juice, I tried to do subtitle conversion, hated it. Had no idea what I was doing, and the end result came out with so many errors anyway.

For the CPU, it may be a slightly related issue as I had over in the previous post. I’m waiting on an i5 10th gen that I’m getting pretty cheaply, going to build myself a new dedicated Plex server to bypass Synology. The CPU I want is pretty close to what Cyber here has, if a gen older. From what I recall, the Passmark score for it was 13K or so, which was barely enough to transcode a 4K non-HDR stream.

This is where I come in. After a lot of back and forth, we seem to have found that subttile burning is a software-only burn-in process. And at least for Synology, it is a single-threaded operation, vs a multithread process. This is the strange part I didn’t get a straight answer to, which is that if you set the server ahead of time to disable hardware transcoding ahead of time then the video transcodes fast enough. Somehow the effort of doing HW transcode, then falling back to SW transcode for subtitle, then back to HW to re-combine seems to cause the CPU to perform much worse than if it did it all in SW in the first place. I bring this up to wonder if it’s happening to you as well, Cybersteel. If you are up for a tiny experiment, disable HW transcoding in the server settings, then try to view the same video. Let me know if it transcodes slowly.

[cybersteel8]

Ah, sorry, my post was originally a comment on another thread about a slightly different topic. I’m not asking for ways to avoid transcoding, I understand I could manually use OCR to convert VOBSUB/PGS subtitles to SRT and all that, that’s not really the issue I’ve noticed.

What I’ve specifically observed is that the Plex Transcoder struggles when burning image subtitles onto a video it would normally cruise through. The speed value drops from 10 to below 1, causing it to buffer.

I would provide more details but I’ve noticed a more severe issue - when I add image subtitles to a stream, it seems to straight-up stop. The video is black, and eventually an error appears saying “Conversion failed. The transcoder exited due to an error.”

I am unsure how to submit more information about what the possible cause could be. Would the video file information be useful? Which logs should I attach, and are they sanitized of any personal data or should I submit only relevant portions?

I would love to provide more information, please tell me what would be appropriate to attach.

[cybersteel8]

Interesting suggestion. I am testing it now, and it appears that using software encoding is significantly slower. The 9-10 I was seeing is now about
4 for the same video file.

Hardware:

Software:

Requesting the transcoder to burn the PGS subtitles onto the video still doesn’t seem to work unfortunately. As soon as I select the subtitle file, the player shows a black screen, time doesn’t tick up, and eventually the error shows. This seems to happen when using hardware transcoding as well.

Furthermore, if I select the subtitle before starting the video (I waited until there were nothing that was still considered playing) there is no entry in Tautulli saying that something is playing. I saw the CPU usage spike up to nearly max on all cores, then it dropped down after a short time, with no results.

[JuiceWSA]

Plexweb can’t be the client.
Plexweb runs in your browser.
Forget Plexweb.
Stop trying to use Plexweb.

If you use one of the Dedicated Player Apps Plex provides - Free, at the Devices Apps links at the bottom of this page - your experience should greatly improve.

The Deal, since you’re running a “Server” that doesn’t have enough horsepower to do much of anything - is you need to be in Direct Play at all times especially when trying to use VOB/PGS/Other Image Based Subtitles. Plexweb is NOT the tool for that job.

Plex for Windows - check
Plex for Mac - check
Plex HTPC - check
Shield - check
Other PGS/VOB Friendly Device and Plexweb is NOT one of them.

I run an entire fleet of Roku Ultras - great devices, cheap, dependable Plex Client with a team of people that can get the job done (in less than years), but unfortunately Rokus vomit a purple stream of awful when confronted with any image based subtitle - so I must endure the OCR conversion - in the somewhat rare instance I can’t find something text based at Subscene that fits:
https://subscene.com/

I don’t need to do the OCR conversion often, but have done it enough to get pretty good at it - while still loathing every moment of it.

Hot Tip for OCRing:
turn off that dam dictionary and ‘Prompt for unknown words’ when doing an entire run of The (original) Twilight Zone (for instance). Using that list of ‘Unknown Words’ you can fix those words each time before you exit the OCR section (so you can see what the trouble is), or you can note the most frequent offenders and add those to the ‘Multiple Replace’ list for more instant gratification AFTER the OCR process is done:

Any Wild Ass Guesses made by SE, are so Wild-assed, they’ll never show up in anything except an OCR Session - so once you create them, you can leave them switched on. They’ll sometimes catch someone else’s ‘mess-up’ - when they didn’t fix 'em, when they sat through that OCR process they hate too.

Since I’m changing THE CAPS LOCK NIGHTMARE, to lower case, SE misses the twilight zone as a proper name so I even fix that - 'cause it annoys me…lol

[cybersteel8]

Changing player is also not a solution to the performance of the transcoder. I have family that use the Chrome browser to watch Plex. I also have other TVs that require the subtitles to be burned in. I fully understand that OCR is an option to not have any image subtitles anywhere - I’ve done it before and I am familiar with the process. I chose the Chrome browser to demonstrate the burn-in behaviour. Avoiding using the transcoder is not a solution, that’s a workaround. I am specifically trying to report a transcoder issue.

[JuiceWSA]

Well then…

You’re using a client that can’t direct play your material.
That will be difficult to get fixed.

Best of luck to you with that Feature Request.

[cybersteel8]

Thanks.

Just to be clear, I am not asking for the players to be able to direct-play the material, unless that is an easier solution to implement than resolving the drop in performance of the transcoder when burning-in subtitles.

The way I see it, if nobody reports issues with the transcoder, the transcoder will never improve. I understand that transcoding is a last resort, but it’s a feature of Plex nonetheless and if I experience problems with it, I hope that the Plex team would appreciate those problems reported to them.

[Divideby0]

One question about your subtitles: Are they internal or external?

Otherwise, I agree. They need to figure out and fix why subtitle processing performs so much worse compared to doing SW transcoding for the entire process.

[cybersteel8]

They are internal. I wonder how much of a different it makes!

I absolutely agree. Software encoding is painfully slow, I don’t even know if overall it is a faster way to transcode, but at least the darn video transcodes at all