ffmpeg官方文档说明

-vsync parameter
Video sync method. For compatibility reasons old values can be specified as numbers. Newly added values will have to be specified as strings always.

0, passthrough
Each frame is passed with its timestamp from the demuxer to the muxer.
1, cfr
Frames will be duplicated and dropped to achieve exactly the requested constant frame rate.
2, vfr
Frames are passed through with their timestamp or dropped so as to prevent 2 frames from having the same timestamp.
drop
As passthrough but destroys all timestamps, making the muxer generate fresh timestamps based rel="stylesheet">