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I've made a mistake and I need to re-encode video from 30 fps to 24fps, while maintaining the same speed/duration.

Does anyone know how I might do this in FFMPEG?

I've tried changing the -r value and this changes the rate, but changes the video length.

Thanks

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    Show the full command you used. – Gyan Mar 10 '17 at 5:01
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To change the frame rate without modifying the total length of the video, FFmpeg has to duplicate or drop some frames. Unfortunately this process can turn a steady smooth movemen to become clumsy and unnatural in the video.

ffmpeg -i input.mov -r 24 -y output.mov

This method is very slow so be patient. The audio will remain the same, since length does not change.

You can also check this tool if your results are not what you expected: MVTools

Here is a thread that may be helpful Change framerate in ffmpeg without re-encoding

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    -sameq has been removed from ffmpeg a long time ago and never meant "same quality". For encoding with x264 use -crf instead. – llogan Mar 10 '17 at 6:52
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    Also, without using a filter, ffmpeg does not interpolate frames. It either duplicates or drops them. – Gyan Mar 10 '17 at 13:17
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This command won't change the video playback speed:

ffmpeg -i <input> -filter:v fps=fps=30 <output>

Worked well for reducing fps from 59.6 to 30.

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