SOLVED: Issues with subtitle syncing

[growntomcat]

When putting particular movies with subtitles on jellyfin, there are moments where the subtitles will completely go off track to a different part of the movie. However when I view the file on my local computer using VLC, the subtitles sync just fine? If it matters, I use MKVToolNix to mix subtitles and video and I export it locally and then upload to my jellyfin.

[TheDreadPirate]

What kind of subtitles are the and what client are you having issues on? Does this issue happen with all clients?

[growntomcat]

They are SRT files, it's my firefox web client, yes it happens on all clients.

[TheDreadPirate]

Are the subtitles external or embedded? Are you able to attach the SRT subs to a forum post so we can examine the contents?

[growntomcat]

I believe they are external as I can toggle them on and off. I don't think I am allowed to attach SRT files so I have created a link to the download: https://files.catbox.moe/4g0vb3.srt
I don't think there's anything wrong with the SRT files as they work perfectly on VLC locally.

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Also an update on the subtitle issue: It doesnt seem to be going off track any more, but when brushing through the film the subtitles will go off sync about 1-2s and generally when watching it they seem to go off.

[Ted Hinklater]

I also noticed subtitle sync issues with subs mainly if I continue watching after a certain time. If I start a 25min show 10mins in I can expect the subs to be a second off and have to use the subtitle offset. Seen it with PGS and ASS subs. Also I feel the offset is backwards; if the subs are a second early, I'd expect to use an offset of "+1s" to offset them by +1s but that makes the subs even earlier. It's more like using the slider to tell JF the subs are "currently at -1s", rather than telling JF to offset the subs by +1s. Hope I explained that half-decently.

[TheDreadPirate]

There are two warnings when I run this sub through a validator.

   
   

I placed the subs in a random movie and these warnings only caused the subs to stack or shift briefly.  So I doubt they are the issue.

[growntomcat]

Hi,
Just wanted to ask for any more thoughts or possible troubleshooting I could do? The problem is persisting still after a couple months. Sorry if this bumps the discussion or something like that, I'm not familiar with how it works.

[TheDreadPirate]

Since SRT subs are directly sent to the client, a lot of the work is done by the client so it is partially/entirely up to how each client handles displaying SRT subs. From my own testing it seems that Firefox loses sync the most easily.

Some preliminary tests I did a while ago, but forgot to follow up on, was that remuxing the video with this command APPEARED to resolve the issue.

Code:

ffmpeg -fflags +genpts+igndts -i "/path/to/video.mkv" -codec copy -max_muxing_queue_size 2048 -max_interleave_delta 0 newFile.mkv

Even though the SRT subs were external in my case, it appeared that the +genpts+igndts options resolved any subtitle desync issue on the test video I had.

[growntomcat]

Yes, that works great! I also noticed that when I was testing today, I was wrong and subtitle syncing actually worked fine on my android TV, but for some reason not on either of my firefox-running desktops.
This works great for correcting the client side subtitle logic.
Ignore what I said before if you saw it, that was my own stupidity. Thank you for your help.