A Look at the MPEG Video Coding Standard for Variable Bit Rate Video Transmission

such as interframe coding combined with variable length codes[9] or conditional replenishment[6] [8], that are less efficient than current coding techniques. Other techniques such as subband coding[1] have demonstrated promise for only a few types of services. Another recent study, which provides insights into long run statistics for coded video sequences, used block based intraframe discrete cosine transform (DCT) coding[3], but the video coder in the study does not utilize any interframe coding. Statistics obtained in these studies, while important, may not be valid for current coding techniques. A source characterization method which models the properties of a large number of sources is ideally required for the purposes of analysis. One approach for achieving such models for video sources is to identify basic components of video sources and video sequences and from these properties predict bit rates for these sources[7]. While this can lead to a universal source characterization framework, the basic components which need to be identified for modeling the source may not be readily available and may vary among different classes of coding algorithms. The parameters needed for such an approach may therefore not be easy to obtain. The other approach is to model video sources where the source coding is based on a standard algorithm which is flexible enough to be applied to a multitude of video services, which is the approach taken in this paper. We study the output stream of a video coder, which complies with the Motion Pictures Expert Group (MPEG) coding standard, with an NTSC quality video sequence as the input. Unlike previous works, we do not examine just bit rates per frame but also the manner in which these bits may arrive within the time period of a frame. This involves examining the bit generation process at the lowest level possible and then applying a packetization algorithm to generate ATM cells. Since the MPEG video coding algorithm has been proposed for a variety of applications, we also investigate the effect of changing some of the coding parameters on the statistics of interest. The outline of the paper is as follows; section 2 presents an overview of the MPEG coding algorithm including the modifications made for our real time application, section 3 discusses modeling issues for the MPEG video source and section 4 describes some of the statistical results obtained. Abstract In this paper we take a close look at the MPEG video coding standard for the transmission of variable bit rate video on ATM based Broadband ISDN. The MPEG standard has been defined for use in a variety of applications. Our focus in this paper is in its use for real time transmission of broadcast quality video. We were particularly interested in observing the impact of two key parameters, the intraframe to interframe picture ratio and the quantization index which are defined in the standard, on the bit rates per frame. These parameters may be used to control video sources depending on the state of the network. Also, as opposed to previous work which looks only at bit rates per frame, we study the bits generated per macroblock, the basic MPEG coding unit. By packetizing these bits, we obtain insights into the cell arrival process to a network for a MPEG video source.