Buffer analysis and control in CBR video transcoding

In video distribution services, various networks of different capacity may be linked together to form the branches of a multicasting tree. In order to efficiently use the available bandwidth of each subnetwork, a video transc oder capable of reducing the bit rate of the incoming bit stream must be us ed at those nodes whose output links have less bandwidth than the input one , In this paper, we analyze the buffering implications of inserting a video transcoder within the transmission path. For transcoders with either fixed or variable compression ratio, we show that the encoder buffer size can be maintained as if no transcoder existed while the decoder has to modify its own buffer size according to both the bit rate conversion ratio and transc oder buffer size. We derive the conditions that have to be met by both the encoder and transcoder buffers for preventing the decoder buffer from under flowing or overflowing. Furthermore, based on the characteristics of consta nt bit rate (CBR) MPEG coded video, we show that efficient bit rate control can be implemented in CBR video transcoders such that the picture quality of transcoded sequences is practically the same as those directly encoded f rom the original sequence at the same reduced bit rates.