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I want to make a video play at 75% speed, so I used the following command:

ffmpeg -i combinedfullspeed.mp4 \
   -filter_complex "[0:v]setpts=1.25*PTS[v];[0:a]atempo=0.75[a]" \
   -map "[v]" -map "[a]" slow3.mp4

However, the video and audio are way out of sync. Any idea why?

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The reciprocal of 75% i.e. 0.75 is 4/3 or 1.3333, so

ffmpeg -i combinedfullspeed.mp4 -filter_complex "[0:v]setpts=(4/3)*PTS[v];[0:a]atempo=0.75[a]" -map "[v]" -map "[a]" slow3.mp4

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