Reduced Storage Transmission Buffer Designs for an Mpeg Video Coder

DESIGNS FOR AN MPEG VIDEO CODER Y S Saw J M Hannah P M Grant Electrical Engineering, University of Edinburgh, Scotland Abstract In MPEG video encoding, e cient bu ering and rate control is especially crucial for constant bit rate (CBR) applications such as non-ATM (Asynchronous Transfer Mode) channels and satellite communication channels. In a CBR environment, compressed video data, which is inherently variable in terms of bit rate, should be throttled to a channel with xed rate by managing the bu er operation. At lower transmission rates, or in the case of an abrupt scene change, a dramatic increase in bu er occupancy or a bu er over ow occurs and this may cause an interruption to normal encoding and consequent degradation of video quality. An intelligent bu ering algorithm is proposed for preventing bu er over ow and for smoothing out the occupancy uctuation. The algorithm exploits major system parameters which have direct in uence on one another in the MPEG encoder. The performance of the proposed algorithm has been veri ed on an MPEG1 encoder. 1 Bu ering in MPEG MPEG, ISO (1) is used for video compression and transmission with constant bit rates of up to about 1.5 Mbit/s which is usually called the sub-primary transmission rate. As the compressed video stream from an MPEG video encoder comprises variable rate tra c there has to be a bu er for converting the variable rate tra c to constant rate tra c in a way which controls the amount of incoming video tra c. Research on bu ering and adaptive quantisation has been carried out, Chen and Wong (2), Lee et al (3), Kawashima (4). Most of this research focuses on adaptive quantisation itself e.g. adaptive bit allocation, Gersho and Gray (5). MPEG is based on inherent system parameters such as picture type, direction of motion compensation, quantisation scale, and quantisation matrices, etc. These parameters are inter-related and this a ects the operation of the encoder. Hence, in this research, more attention is given to these inherent system parameters. The key parameters which control the tra c rate are quantisation scale and quantisation matrices, ISO (1), especially, quantisation scale which directly controls the bu er occupancy. As the number of coded bits of the encoder uctuates with the other parameters such as picture type as well as picture details, the encoder generates a compressed video bit stream with periodic changes. Eventually, the occupancy shows repetitive variation. This is an important system parameter to be considered in bu ering. The bu er control mechanism de ned in MPEG gives basic guidelines for avoiding interruptions to bu er operation or abrupt change of video quality. They are a few qualitative guidelines as follows; allocating coded bits based on picture type controlling quantiser step size proportional to bu er occupancy controlling quantiser step size inversely proportional to the amount of data in pictures discarding high frequency DCT (discrete cosine transform) coe cients when over ow occurs However, the actual bu ering algorithm depends on the designer as there are no quantitative speci cations in the MPEG standard. Figure 1 shows the schematic con guration for bu ering in MPEG