参考网页
https://trac.ffmpeg.org/wiki/How%20to%20speed%20up%20/%20slow%20down%20a%20video
ffmpeg -i test.mp4 -filter_complex "[0:v]setpts=0.8*PTS[v];[0:a]atempo=1.25[a]" -map "[v]" -map "[a]" -b:v 400K -b:a 32k -c:v libx264 -s 1280x720 2_1280_720_264.mp4
上诉命令快放1.25倍，即将原来的视频时长缩短为80%.