Easily fix async video with ffmpeg

If you have an constantly asynchronous video you can easily fix it with one of the following two commands:

Case 1: Audio ahead of video:
ffmpeg -i input.flv -itsoffset 00:00:03.0 -i input.flv -vcodec copy -acodec copy -map 0:1 -map 1:0 output_shift3s.flv

Case 2: Audio behind video:
ffmpeg -i input.flv -itsoffset 00:00:03.0 -i input.flv -vcodec copy -acodec copy -map 1:0 -map 0:1 output_shift3s.flv

The difference is in the mapping parameters which specify which of the two supplied input files to map on which output channel. The "-itsoffset" option indicates an offset (3 seconds in the example) for the following input file. The input file is required to have exactly one video channel at position 0 and one audio channel at position 1.

I added "-vcodec copy -acodec copy" to avoid reencoding the video and loose quality. These parameters need to be added after the second input file and before the mapping options. Otherwise one runs into mapping errors.