Demux, mux and cut MP4 in ffmpeg

With the upcoming COVID-19 open-source conferences season, we record presentations and screencasts almost on daily basis. Sometimes, it’s needed to trim a MP4 video without reencoding the content. It’s easy with ffmpeg, option --s specifies start in format of 00:00:00.000 or just 0 as number of seconds, option -t represents length of the desired section:

ffmpeg -ss 0 -i video-orig.mp4 -t 00:20:51.000 -c copy video-cut.mp4

Sometimes video and audio needs to be separated into individual files (aka demuxed). This can be handy when some audio artifacts need to be removed (e.g. noise or buzz) from the audio track (aka stream). This can be done easily:

ffmpeg -i video-orig.mp4 -an -vcodec copy video-demuxed.m4v
ffmpeg -i video-orig.mp4 -vn -acodec copy audio-demuxed.m4a

Note that m4v and m4a are not well known but standard extensions for MP4 audio and video streams. If you click on such file, most modern operating systems should play the file, assuming it was encoded with a codec a player understands (e.g. h265 or h264 for video and AAC for audio).

Sometimes, audio needs to be extracted into WAV format rather than M4A for further processing. The rate can be either 44100 or 48000 depending on the source, if you don’t know just use 48000:

ffmpeg -i video-orig.mp4 -vn -acodec pcm_s16le -ar 48000 -ac 2 audio-demuxed.wav

After corrections are made, let’s say in Audacity software, audio can be compressed back via AAC codec:

ffmpeg -i audio-edited.wav -c:a aac -b:a 128k audio-edited.m4a

And finally, audio and video can be joined (muxed) back together. Note that at any point, video stream hasn’t been modified (encoded) so no quality is lost during this process. In the example above, only audio was reencoded back into AAC which is fast. Muxing and demuxing is also matter of few seconds:

ffmpeg -i video-demuxed.m4v -i audio-edited.m4a -c:v copy -c:a copy video-final-version.mp4

All of this will also work for other video containers like MKV. This little tool named ffmpeg is indeed a monster.

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