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# ffmpeg
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# FFmpeg
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[ffmpeg](https://www.ffmpeg.org) is a free and open-source suite consisting of
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[FFmpeg](https://www.ffmpeg.org) is a free and open-source suite consisting of
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many audio and video tools and libraries.
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## Usage
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@ -32,3 +32,34 @@ ffmpeg -f concat -safe 0 -i files.txt -map 0 -c copy output.mp4
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If the video files you want to concatenate are not mp4 files change the above
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command accordingly.
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### Two-Pass Encoding
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Two pass encoding - as described in the
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[official FFmpeg documentation](https://trac.ffmpeg.org/wiki/Encode/H.264#twopass)
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- uses two passes.
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The first pass analyzes the input data and outputs a descriptor file.
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The second pass actually encodes the data.
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The following is an example where the file `input` is encoded with `libx264` to
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`mp4` video with `libfdk_aac` audio.
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The video bitrate is `555k` and the audio bitrate is `128k`.
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```sh
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ffmpeg -y -i input -c:v libx264 -preset medium -b:v 555k -pass 1 -c:a libfdk_aac -b:a 128k -f mp4 /dev/null && \
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ffmpeg -i input -c:v libx264 -preset medium -b:v 555k -pass 2 -c:a libfdk_aac -b:a 128k output.mp4
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```
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### Encode Audio/Video to Target Size
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For the encoding of a file to a target size the target bitrate of the output
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video is needed.
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An explanation of this was given by
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[aergistal on Stack Overflow](https://stackoverflow.com/questions/29082422/ffmpeg-video-compression-specific-file-size).
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This can easily be done with the calculation `bitrate = target size / duration`
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in Bits/Second.
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Afterwards the encoding can be done by using Two-Pass Encoding as explained in
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[a previous section](#two-pass-encoding).
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Note that the bitrate for videos is split amongst a bitrate for video and a
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bitrate for audio.
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The target bitrate has to be equal to or greater than the sum of both video
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bitrate and audio bitrate.
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