The bandwidth flexibility offered by the asynchronous transfer mode (ATM) technique makes it possible to select picture quality and bandwidth over a wide range in a simple and straightforward manner. A prototype model of a video codec was developed that demonstrates the feasibility of both variable bit rate (VBR) coding and user-selectable picture quality. The VBR coding algorithm is discussed and it is shown how a stabilized quality is achieved and how this quality and associated bandwidth can be selected by the user. How error propagation is limited to reduce the visibility of cell losses is also discussed. Interfaces with the ATM network are analyzed, with emphasis on decoder synchronization and absorption of cell delay jitter. The VBR codec offers very good picture quality for videophony applications at an equivalent load of 5.9 Mb/s. Picture quality remains relatively constant, even for heavy motion.>

The induction of the asynchronous transfer mode (ATM) concept may significantly influence the coding of video services for broadband networks. The authors show how the absence of a physical channel structure and the ability to switch bursty traffic can be used to enhance video coding. Packetization defects and their impact on picture quality, coding algorithms, and synchronization schemes are studied. The authors describe variable-bit-rate coding and report on the results obtained with a hardware implementation of a variable-bit-rate video codec. Statistical multiplexing gain figures are given. The influence of cell loss on image quality is discussed and simulation results are given. A layered coding model offering good cell loss concealment properties and high flexibility is described.>