Adjusting Subtitles / Closed Captions sync after removing commercials

subtitles
commonissues

(Administrator) #1

Sometimes after cutting commercials in the final output file the subtitles/closed captions go out of sync with the audio. You can correct it by using this procedure.

There are two types of sync issues:

  1. Constant offset - where the subtitles are out of sync by a constant value for the entire duration of the recording (e.g. 2.5 seconds ahead). This corrected by adjusting the Subtile Offset (under Conversion Task Settings -> Advanced Settings -> Expert Settings). This number can +ve or -ve depending upon whether the subtitles are leading or lagging the audio.

  2. Progressive offset - where the subtitles get increasing out of sync after each commercial cut (e.g. starts with 2.5 seconds and then after each commercial cut it adds another 2 seconds, so after the 4th commercial cut it is 2.5 + 2*4 = 10.5 seconds ahead) This can be corrected by adjusting the Segment Offset (Under Settings -> General Settings -> System Settings). This number can +ve or -ve depending upon whether the subtitles are leading or lagging the audio.

This is a bit of trial and error. You may have to use a combination of Subtitle Offset and Segment Offset parameters to correct it completely.

Reason and extend of why this happens depends heavily on the video recording and type of encoder being used. Video files typically have group of frames and boundaries (e.g. I Frames). FFMPEG and Handbrake typically cut along boundary lines. Depending upon how the original video was encoded the boundary lines can appear every few seconds, so if MCEBuddy asks ffmpeg to cut on the 30th second, ffmpeg will select the next boundary line which would be 32 seconds and cut there, where as the Subtitles will be cut/adjusted at the 30th seconds which leads to this sync issue. Videos with more I Frames and smaller group of frames will cut more accurately.

###NOTE:

  • Segment Offset one allows you to compensate for the subtitle sync DRIFT (+ve or -ve) after EACH commercial is cut.
  • Subtitle Offset allows for a ONE time shift (sync) at the beginning of the video (+ve or -ve)

Subtitles out of sync - how to do this workaround?
Subtitles out of sync - how to do this workaround?
(Administrator) #2