tbn
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tbn: the time base in AVStream that has come from the container
视频流时长的精确度time_base，即1s = 90000个刻度
视频总时长T = AVStream.duration * AVStream.time_base
tbc
- tbc: the time base in AVCodecContext for the codec used for a particular stream
编解码的时间精确度time_base, 即 1s = 48个刻度，和视频的pts和dts有关。
视频的显示时刻Tpts = AVFrame.pts * AVCodecContext.time_base
视频的解码时刻Tdts = AVFrame.dts * AVCodecContext.time_base
tbr
- tbr: tbr is guessed from the video stream and is the value users want to see when they look for the video frame rate
每秒显示图片的数量，24 tbr 也就是帧率的相同表示：24 fps