/ Anyone know anything about MP4 file processing?

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Martin W 09 Jan 2020

This is a rather abstruse problem but I know UKC has a tendency to conceal unexpected sources of knowledge and expertise so here goes...

I have an MP4 video file, and a .srt subtitles file.  Using VLC to play the video, it finds the .srt file and the subtitles are nicely in sync with the audio.  However, the video playback apps on some of my devices can't use .srt files, so I'm trying to use Handbrake to embed the subtitles in the MP4 file.  The trouble is that Handbrake seems to find a few seconds of additional video at the start of the MP4 file, which it includes in the output MP4 file, which then puts the embedded subtitles out of sync by the duration of the additional bit of video.

I've tried editing the extraneous video out of the MP4 file but somehow Handbrake keeps finding it and including it.  It's as if the tools I'm using the edit it out don't actually remove the video, they just put a tag in the MP4 file saying "start here", which Handbrake then ignores.

Anyone have sufficient knowledge of this subject area to make any informed suggestions as to what might be going on and/or how to stop it happening?

Push comes to shove I suppose I could edit the .srt file to bump up the trigger times for the subtitles by the duration of the extraneous video - but that looks like it would be a fair old job for over two hours of video

Iamgregp 09 Jan 2020
In reply to Martin W:

I'm no expert in these areas but it sounds like the extra few seconds might be due to the guardbands setting on whatever you're using to edit it?  Maybe check that these are set to zero? 

freeflyer 09 Jan 2020
In reply to Martin W:

I have no expertise whatever in this, but how about using the trim function in Handbrake to remove the first few seconds?

Luke90 09 Jan 2020
In reply to Martin W:

According to the Handbrake docs:

"You can set an offset (measured in milliseconds) to change the start time that the first and subsequent SRT subtitles will appear. Use trial and error encoding a single chapter to obtain the correct offset."

If you already know, or can measure, the length of that added chunk of video, you should be able to skip the trial and error part.


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