Incorrect mov_text subtitle timing after a long subtitle-free interval

meckey

I found a subtitle timing issue when downloading the Amazon movie “Backdraft.”

I compared the original Japanese forced-subtitle TTML file with the mov_text subtitle in the downloaded MP4. The timestamps match through subtitle 25, but an approximately 80-minute subtitle-free interval is lost.

Correct TTML timestamps:

25 00:49:44.857 → 00:49:50.029
26 02:10:18.102 → 02:10:21.606
27 02:10:21.731 → 02:10:28.112

Timestamps in the MP4 mov_text subtitle:

25 00:49:44.857 → 00:49:50.029
26 00:49:50.029 → 00:49:53.533
27 00:49:53.533 → 00:49:59.914

The subtitle durations remain correct, but the 01:20:28.073 interval between subtitles 25 and 26 becomes zero, shifting the remaining subtitles forward.

The TTML-to-mov_text conversion or MP4 muxing process may not handle very long subtitle packet durations correctly.

Please investigate and ensure that the absolute subtitle timestamps from the TTML file are preserved.

p.s
After further testing, I found that the timestamps are correct when the subtitles are downloaded as an SRT file. Could the problem be occurring when the subtitles are muxed into the MP4?

Germania

I personally can’t test the title at amazon.co.jp ..
.. and at amazon.com are no forced (or jp) subtitles

meckey correct when the subtitles are downloaded as an SRT file.
Could the problem be occurring when the subtitles are muxed into the MP4?

You could try remuxing subs using the video format mkv(MKVToolNix)

btw: You might find some anomalies in your StreamFab.log ..
.. if in doubt, you could always attach it.

darth_ep1

meckey I found a subtitle timing issue when downloading the Amazon

I can confirm that. Same issue with “Lee Cronin’s The Mummy” here. The German FORCED subtitles do have a timeshift and start to appear too early in the movie during the second half of the movie.

The same applies to the movie “King’s Land (Bastarden)” starring Mads Mikkelsen. The FORCED subtitles are shifted and appear to early in the movie - I hat to switch them off, because its distracting while wachting the movie.

download Format: MP4, remux

0xFeedBeef

As far as I recall, AMZN subtitles are single ttml files, so what happens when you download them separately (extract original format) and mux-in yourself?

meckey

The same issue occurs not only with Amazon, but also with Disney+. The TTML files and separately downloaded SRT files from Amazon are correct. Apparently, when FFmpeg muxes subtitles into an MP4 file, it cannot handle excessively long gaps between subtitles (perhaps 30 minutes or more?) and discards them as errors. I don’t think this issue occurs with MKV files, so I’ve decided to switch to that format.

juniper86

This is not a StreamFab problem. It is a known issue with ffmpeg and embedded text in mp4.
If you have forced subtitles with large gaps, then use MKV or external SRT.

There is a manual workaround if you edit the SRT and remux the ffmpeg mp4 yourself.
Just add dots to fill in the gaps. Make the display time so short the dots are not noticeable.
For example, if the first subtitle starts at 30 minutes, add dots every 5 minutes.

:1
00:05:00,000 --> 00:05:00,100
.

2
00:10:00,000 --> 00:10:00,100
.

3
00:15:00,000 --> 00:015:00,100
.

4
00:20:00,000 --> 00:20:00,100
.

5
00:25:00,000 --> 00:25:00,100
.

6
00:30:00,000 --> 00:30:05,000
FIRST SUBTITLE

darth_ep1

juniper86 If you have forced subtitles with large gaps, then use MKV or external SRT.

Thank you, now I have learned something, again.
I already saw that subtilte-problem quite a while ago, but did not know why this happens. Blamed my playing-devices for that… …now I know it is a ffmpeg issue.

That means: If you download any content containing FORCED subtitles, use MKV instead of mp4…. ….will stick to that.