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.