提取指定stream
time avconv -i i.mkv -map 0:0 -map 0:1 -map 0:5 -c:v copy -c:a:0 mp3 -c:s copy o.mkv
合并
avconv -i i1.mkv -i i2.mkv -map 1:0 -map 1:1 -map 0:6 -c copy o.mkv
改变profile以改变视频大小(x264)
time avconv -i i.mkv -c copy -c:v libx264 -profile:v main o.mkv