#srtresync 1.0
srtresync is a simple, straight-forward, and powerful utility used to correct ("resync") drift in srt subtitle files.
srtresync is developed and maintained by Mahmoud Al-Qudsi <mqudsi@neosmart.net> on behalf of NeoSmart Technologies, and is released completely open source under the terms of the MIT license. See the LICENSE file for further information.
##Modes of Operation
Unlike most other utilities, srtresync can be used to correct both a fixed offset (say subtitles are consistently five and a half seconds behind) or linear drift (the difference between the subtitles and the audible dialog changes at a fixed rate, so that the difference/delay between audio and subtitles at the start of a film or clip is different than at the end).
###Correcting Fixed Offset
In its simplest mode of operation, srtresync can be used to add or subtract a constant amount from the current display times for a .srt subtitle file. Usage in this mode is exceedingly straightforward:
srtresync ./input.srt [+/-]hh:mm:ss.xxx
This will lead srtresync to read the current subtitles from ./input.srt and to write the output to the console, adjust forwards or backwards by hh:mm:ss.xxx hours/minutes/seconds/milliseconds (the +/- can be omitted if adding).
###Correcting Linear Drift
A common anomaly is to have a subtitle file which starts off either in-sync with the spoken audio track or deviating from it by only a fixed offset, but then to have the difference between the two either increase or decrease at a constant pace, such that by the end of the clip the difference is no longer whatever it started out as.
srtresync can handle this quite easily, requiring you to only identify two differences from which it will derive the data needed to calculate and adjust for linear drift. For best results, a difference from the beginning of the clip/film and one from towards the very should be used (to maximize precision):
srtresync ./input.srt hh:mm:ss.xxx-hh:mm:ss.xxx hh:mm:ss.xxx-hh:mm:ss.xxx
For example, if you have a clip and corresponding srt where the subtitles currently displaying at 00:00:10.500 should actually be shown at 00:00:07.200 and the subtitles showing at 02:14:03.007 should actually be shown at 02:22:17.400, then you would execute srtresync as follows:
srtresync ./input.srt 00:00:10.500-00:00:07.200 02:14:03.007-02:22:17.400
srtresync will automatically calculate the linear drift parameters and adjust the output accordingly. In addition, it'll print the calculated variables to stderr for use as a quick sanity check:
Offset will be 00:00:03,950 scaled by time at a rate of -0.06195986
Indicating that the initial offset was determined to be 3.950 seconds, and that as the movie goes on, this value will adjust with a linear coefficient of approximately -0.062 with regards to the time (in milliseconds). This makes sense, because we start off with with the subtitles 3.3 seconds behind and end with the subtitles being 494.4 seconds ahead so our positive initial offset needs to shrink towards zero and then beyond as the film progresses.