ffmpeg转换文件格式

• pcm->mp3
  https://blog.51cto.com/u_14202100/5081305

• mp4->yuv
  ffmpeg -i input.mp4 -pix_fmt yuv420p output.yuv

• ffmpeg无法将mp4的音频格式转码成pcm_s16le或者pcm_s16be格式
  ffmpeg -i test3.mp4 -vcodec copy -acodec pcm_s16le test3_2.mp4执行报错：

[mp4 @ 0x26a5f730] Could not find tag for codec pcm_s16le in stream #1, codec not currently supported in container