随着数字音视频及多媒体技术的发展，迫切需要压缩巨大的视频信号数据，适用于不同领域的MPEG-1、MPEG-2、MPEG-4、H．261、H．263、H．264等国际编码标准应运而生。为了适应用户终端的多样性以及网络自身的传输特性，常需将已压缩的视频码流进行转换编码，以实现网络间资源共享。主要介绍了对已有的压缩视频流根据目标的要求如何进行相应的变换，如通过改变量化参数获得码率的变换；通过下变换获得缩小的图像，实现空间分辨率的变换，以及跳帧获得时间分辨率的变换。对不同变换器结构形式进行了分析，以获得结构复杂性和图像质量的均衡，并重点剖析了转换编码器核心技术——运动矢量的确定。

Along with digital AV and multimedia technology developing, large numbers of video datum need to be compressed. At present ,the International Coding Standards, such as MPEG-1, MPEG-2, MPEG-4, H.261, H.263, H.264 have been established and applied to different application areas. In order to adapt to heterogeneous client terminals and transmission characteristic of networks, pre-encoded video should be transcoded into another coding format. So, the resources of networks can be shared. The content of this paper is about how to convert a previously compressed video bitstreams into a lower bit-rate video bitstreams according to the requirements of heterogeneous client terminals, such as bitrate transform by changing quantizer parameter, spatial resolution transform by down-sampled, temporal resolution transform by skipping frames. In this paper the architectures and mechanism of transcoder are analyzed and the best tradeoff between quality and complexity are offered. Also the core technique: how to obtain the motion vector is anatomied.