Transcoder drops significantly during subtitles

[Plexus1]

Server Version#: 1.18.7.2457
Player Version#: 4.37.2 (plexweb)

Having a nightmare with subtitles. My plex server runs hardware decode and transcode. The following appear in plex logs in verbose during an average 4k source file transcode

vdec_hw_status => 1
speed => 4.4 (bounces around 4)

As soon as I enable subtitles on the client, it buffers relentlessly and speed goes to

speed => 0.3

Does this on all 4k source videos.

1080 h264 source files :

speed => 7.9 (between 6 and 8)

1080 h264 with subtitles (vobsub) enabled

speed => 1.3

So it seems that some significant slowdown on several orders of mangnutude is happening with subtitles selected. my GPU and CPU are happy to keep up for 1080 h264 content, but subtitles on 4k content make it unwatchable (even though the GPU is happy do 4x speed without the subtitles).

any thoughts?

[JuiceWSA]

All kinds of reasons why Image Based Subtitles cause issues… have a look here:

[INFO] Plex, 4k, transcoding, and you - aka the rules of 4k

NOTE: for 2022 – Plex has come a long way since this FAQ was originally written, HW transcoding has become more available and more stable, and tone mapping was recently added to address the hdr/sdr color conversion issues. The first 4 ‘rules’ generally are no longer as important as they once were, but may still be a good thing to bear in mind. Dolby Vision, is an entirely other mess. THIS thread/FAQ is NOT about DV. DV work is still in progress, and there are no perfect TV’s or external devic…

tl;dr:

Find some srt/text subs that’ll Direct Play:

Subscene - Passionate about good subtitles

[Plexus1]

Thanks very much. Its weird as fair enough - transcoding 4k is always going to have issues.

So let me reword it - if instead transcode 2 streams of 1080 h264 then I get speed 4 for each stream. If I then enable subtitles both buffer and speed goes to 0.7

So while I do appreciate the 4k guidance, the issue im trying to get at is, why the massive overhead of simply adding subtitles in to transcoded 1080 h264 content?

[JuiceWSA]

Yea, well - there is a reason that linked thread is too long to read.
There are A LOT of reasons 4K transcoding is troublesome.
A LOT.

In a nutshell: there are some things the GPU can help out with and there are some things it can’t help out with. I guess we’re into an area that’s not only difficult for the GPU, but the CPU as well. The performance rating clearly shows there’s a crisis in progress.

The immediate fix is:
create/acquire srt/text subs that will Direct Play.

The PITA fix is - wait for someone that can help you.

I would have already used the immediate fix - and I have - 'cause I don’t have any image based subtitles to cause issues…lol

@TeknoJunky should be along shortly to better explain ‘The Bad News’ - or possibly offer another immediate fix.

[Plexus1]

@JuiceWSA

ere is a reason that linked thread is too long to read.
There are A LOT of reasons 4K transcoding is troublesome.
A LOT.

OK sure, i get that. happy to ditch the whole 4k question. lets concentrate on 1080 h264 instead.

Does ffmpeg suffer the same performance penalty when burning in subs on a VAAPI transcode?

[FordGuy61]

Burning in subtitles involves the CPU, even when using hardware accelerated transcoding.

See quote below from a post in the Synology section of the forum. Synology NAS also use VAAPI for hardware accelerated transcoding.

Other posts mention that burning subtitles is single-threaded, which would also impact overall transcoding speed.

@ChuckPa

Intel never gave us the ability to burn in the images with the hardware. Theoretically, it can do it but no software exists to do it yet. Maybe someday? It will take someone at Intel or like that to write such software.

@trumpy81

Unsupported subtitles must be burned into the video stream and that requires the CPU to process the subtitle in software, regardless of whether or not the video is being processed in hardware.

Subtitle handling is also a single threaded process, meaning only a small portion of the CPU’s processing power can be utilised for this process, approx. 25% to be precise.

[JuiceWSA]

I’ve just added you to my list of folks that can give out ‘The Bad News’…

I’ll just continue with the Good News:
https://subscene.com/
(been there twice today myself - successful both times - and yes - any subs will work as long as they’re from the same type: HDTV/Web/BluRay or DVD usually interchange)

Popular titles can be found at the:
Search Feature - in the pre-play settings menu - or while playing at the same place…
they may even work and if they do, all the subs acquired through the Plex Pass Feature will be srt/txt and Direct Play. Depending on the client, timing adjustments may make them perfect, may not.

[Plexus1]

Thanks folks for the replies and info. appreciated.