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 on frame-rate.
-1, auto
Chooses between 1 and 2 depending on muxer capabilities. This is the default method.

Note that the timestamps may be further modified by the muxer, after this. For example, in the case that the format option avoid_negative_ts is enabled.
With -map you can select from which stream the timestamps should be taken. You can leave either video or audio unchanged and sync the remaining stream(s) to the unchanged one.

理解：

• passthrough 每一帧从解码器到编码器，时间戳保持不变
• cfr 如果指定了输出帧率，输入帧会按照需要进行复制（如果输出帧率大于输入帧率）或丢弃（如果输出帧率小于输入帧率）
• vfr 输入帧从解码器到编码器，时间戳保持不变；如果出现相同时间戳的帧，则丢弃之
• drop 同 passthrough，但将所有帧的时间戳清空

参考

http://ffmpeg.org/ffmpeg-all.html#Advanced-options