将视频从 mp4 转换为 flv 时，我遇到了奇怪的持续时间问题。还有一个不同的“start_time”，那是什么？start_time 是什么意思？
input duration - 0:00:29.577000
input start_time - 0:00:00.000000
output duration - 0:00:51.518000
output start_time - 0:00:21.840000
ffmpeg 命令：
ffmpeg -y -i 18579-1386758783533.mp4 -c:a libfdk_aac -b:a 128k -ar 22050 -c:v libx264 -pix_fmt yuv420p -s 1920x1080 -framerate 30 v18579-1386758783533.flv
FFprobe - 输入文件：
dead@dead-work-desktop:~/36369/wowzaoutput$ ffprobe 18579-1386758783533.mp4
ffprobe version git-2013-11-29-0d640c0 Copyright (c) 2007-2013 the FFmpeg developers
built on Nov 29 2013 11:54:34 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
configuration: --extra-libs=-ldl --enable-gpl --enable-libass --enable-libfdk-aac --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libspeex --enable-libtheora --enable-libvorbis …