How to slow down a video and its audio by 20% using FFMPEG or HandBrakeCLI?

Question

I am trying to slow down a video+audio a little bit, using this FFMpeg command:

ffmpeg.exe -i video.mp4 -filter_complex "[0:v]yadif,framerate=30,setpts=1.2*PTS[v];[0:a]atempo=0.8[a]" -map "[v]" -map "[a]" -vsync vfr -vcodec h264_qsv out.mp4

but this command gives lots of warnings like this:

[mp4 @ 00000194a7089040] Non-monotonous DTS in output stream 0:0; previous: 2002, current: 2002; changing to 2003. This may result in incorrect timestamps in the output file.

and the video and audio are out of sync on the out.mp4

How can I slow down a video and audio using FFMpeg or HandBrakeCLI ?

Answer 1

This seems to work well:

ffmpeg.exe -r 24 -i video.mp4 -filter_complex "[0:v]yadif[v];[0:a]atempo=0.8[a]" -map "[v]" -map "[a]" -vsync vfr -vcodec h264_qsv out.mp4

Source: #handbrake channel on Freenode, user BradleyS