How to slow down a video without slowing down the audio?

I have a security camera that records a 1 min video and compresses it to 40 seconds. The audio is ok. How do I slow down the video from 1.5x to 1.0x without altering the audio?

Using a player to slow down a video without slowing down the audio

You can avoid having to re-encode by having your player slow down the video. Example using mpv:

mpv --vf=setpts=1.5*PTS input.mp4

Using ffmpeg to slow down a video without slowing down the audio

ffmpeg -i input.mp4 -vf "setpts=1.5*PTS" -c:a copy output.mp4

• This re-encodes the video.
• No need to separate audio and video beforehand.
• The audio is stream copied, so audio will not be altered.

More info

• setpts filter documentation
• FFmpeg Wiki: Speeding up / slowing down video