Transcoding of single-layer MPEG video into lower rates

Forthcoming interactive video services such as video on demand will use pre-encoded bit streams for transmission. A great lack of flexibility arises when heterogeneous networks are used or when the user is allowed to use a bandwidth of his/her own choice. In the paper, a mechanism capable of decoupling video encoders from transmission-network constraints and user demands is proposed. The authors devise a low-cost, low-delay video transcoder capable of providing transmission flexibility to pre-encoded bit streams, by reducing their bit rates according to either channel capacity or user demand. Simple techniques such as open-loop coarse requantisation or drop of high-frequency transform coefficients are shown to be inefficient, because of the drift introduced in the transcoded pictures. It is shown that transcoded pictures are drift-free and their quality, on average, is only about 1 dB worse than those directly encoded at the same bit rate. The proposed transcoder is far less complex than a cascade of decoder-encoder, while the picture quality is shown to be better for almost all frames