Combining audio and video files with FFmpeg

When working with various types of video analysis, I often end up with video files without audio. So I need to add the audio track by copying either from the source video file or from a separate audio file. There are many ways of doing this. Many people would probably reach for a video editor, but the problem is that you would most likely end up recompressing both the audio and video file. A better solution is to use FFmpeg, the swizz-army knife of video processing.

As long as you know that the audio and video files you want to combine are the same duration, this is an easy task. Say that you have two video files:

  • input1.mp4 = original video with audio
  • input2.avi = analysis video without audio

Then you can use this one-liner to copy the audio from one file to the other:

ffmpeg -i input1.mp4 -i input2.avi -c copy -map 1:v:0 -map 0:a:0 -shortest output.avi

The output.avi file will have the same video content as input2.avi, but with audio from input1.mp4. Note that this is a lossless (and fast) procedure, it will just copy the content from the source files.

If you want to convert (and compress) the file in one operation, you can use this one-liner to export an MP4 file with .h264 video and aac audio compression:

ffmpeg -i input1.mp4 -i input2.avi -c copy -map 1:v:0 -map 0:a:0 -shortest -c:v mpeg4 -c:a aac output.mp4

Since this involves compressing the file, it will take (much longer) than the first method.

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Alexander Refsum Jensenius is a music researcher and research musician living in Oslo, Norway.