FFmpeg concat video and audio out of sync

Question

Joining multiple files using ffmpeg concat seems to result in a mismatch of the timestamps or offsets for the audio. I\'ve tried with several videos and noticed the same pro

Answer 1

I encountered a similar problem and found a solution that worked, at least for me. In my case, I was also concatenating files, and found audio/video sync problems with iOs, but not with Windows (e.g., VLC media player showed no synchronization problems using the same mp4 file). The symptom for iOs playing this concatenated mp4 was initially good synchronization followed by an increasing loss of synchronization as the movie played, with audio going faster than video. Interestingly, the sync could be restored temporarily by advancing the movie progress slider to any point in the movie, but then the sync would be lost again as the movie continued to play in iOs. By playing the same movie simultaneously in both iOs and Windows VLC, and initially synchronized with each other as well as I could, by observing the evolution of the "echo" between them, I concluded that the iOs audio was going too fast (assuming the Windows player is correct).

For me, the solution was to add the audio filter option -af aresample=async=1000 to the ffmpeg command, which I found as an example in the ffmpeg online documentation and used verbatim. I don't know if this setting is optimal, but the result was a mp4 with audio and video remaining synchronized when played by both iOs and VLC. This ffmpeg option yielded proper iOs synchronization both during concatenation and afterwards when re-encoding the already concatenated file.