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by Mathieu Poliquin

This guide for how to mix audio and video with ffmpeg and fix out of sync problems

First get audio and video total time

ffprobe -i video.mp4 -show_format | grep duration
ffprobe -i audio.mp4 -show_format | grep duration

Devide audio length by video length and set the atempo audio filter variable, in this example 0.999

ffmpeg -i video.mp4 -i audio.mp4 -c:v copy -c:a aac -af atempo=0.999 output.mp4

I made a script to do that automatically example usage: script_name.sh video.mp4 audio.mp3 output.mp4

VIDEO_LEN=$(ffprobe -loglevel 0 -i $1 -show_format | grep duration)
AUDIO_LEN=$(ffprobe -loglevel 0 -i $2 -show_format | grep duration)



RATIO=$(echo "$AUDIO_LEN / $VIDEO_LEN" | bc -l)

echo $RATIO

ffmpeg -i $1 -i $2 -c:v copy -c:a aac -af atempo=$RATIO $3
tags: ffmpeg - audio - video - out of sync