1. Correcting Audio that is too slow/fast
This can be done using the
-async parameter of ffmpeg which according to the documentation “Stretches/squeezes” the audio stream to match the timestamps. The parameter takes a numeric value for the samples per seconds to enforce.
ffmpeg -async 25 -i input.mpg <encoding options> -r 25
Try slowly increasing the -async value until audio and video matches.
2. Auto-Correcting Time-Shift
2.1 Audio is ahead
When audio is ahead of video: As a special case the
-async switch auto-corrects the start of the audio stream when passed as
-async 1. So try running
ffmpeg -async 1 -i input.mpg <encoding options>
2.2 Audio lags behind
Instead of using
-async you need to use
-vsync to drop/duplicate frames in the video stream. There are two methods in the manual page “-vsync 1” and “-vsync 2” and an method auto-detection with “-vsync -1”. But using “-map” it is possible to specify the stream to sync against.
ffmpeg -vsync 1 -i input.mpg <encoding options> ffmpeg -vsync 2 -i input.mpg <encoding options>
Interestingly Google shows people using
-vsync together. So it might be worth experimenting a bit to achieve the intended result :-)
3. Manually Correcting Time-Shift
If you have a constantly shifted sound/video track that the previous fix doesn’t work with, but you know the time shift that needs to be corrected, then you can easily fix it with one of the following two commands:
3.1 Audio is ahead
Example to shift by 3 seconds:
ffmpeg -i input.mp4 -itsoffset 00:00:03.0 -i input.mp4 -vcodec copy -acodec copy -map 0:1 -map 1:0 output_shift3s.mp4
Note how you specify your input file 2 times with the first one followed by a time offset. Later in the command there are
-map parameters which tell ffmpeg to use the time-shifted video stream from the first
-i input.mp4 and the audio
stream from the second one.
I also added
-vcodec copy -acodec copy to avoid reencoding the video and loose quality. These parameters have to be
added after the second input file and before the mapping options. Otherwise one runs into mapping errors.
3.2 Audio lags behind
Again an example to shift by 3 seconds:
ffmpeg -i input.mp4 -itsoffset 00:00:03.0 -i input.mp4 -vcodec copy -acodec copy -map 1:0 -map 0:1 output_shift3s.mp4
Note how the command is nearly identical to the previous command with the exception of the
-map parameters being switched.
So from the time-shifted first
-i input.mp4 we now take the audio instead of the video and combine it with the normal video.