The USC Andrew and Erna Viterbi School of Engineering USC Signal and Image Processing Institute USC Ming Hsieh Department of Electrical Engineering University of Southern California

Technical Report USC-SIPI-332

“Rate Control Algorithims For Low Variable Bit Rate Video”

by Hwangjun Song

May 1999

The demand of multimedia communications is increasing very fast. Visual communication is one main component of a multimedia communication system, which becomes more feasible these days due to the rapid development of network/communication technologies. Since the amount of video data is usually quite large, the limited channel bandwidth is the major bottleneck in a video communication system. As a result, effective video compression techniques are urgently needed. Many international standards such as MPEG-1,2,4, H.263 and H.263+ have been established, and new standards such as H.263++ and H.26L are under current development for higher coding efficiency and more functionalities. With these standards in place, a video codec can be implemented in either software or hardware to achieve the video compression function. One module in the video encoder that is not standardized but plays a key role is the associated rate control scheme. Rate control regulates the coded bit stream to satisfy some given channel and buffer conditions and, in the same time, attempts to optimize the quality of coded video under these constraints. In this thesis, we examine several novel rate control algorithms for various application environments. The thesis work consists of three main results. First, we consider a novel H.263+ compatible encoder that consists of two major components, i.e. a variable encoding frame rate method and a hybrid DCT/wavelet I-frame coding scheme for low bit rate video coding. Most existing rate control algorithms for low bit rate video focus on bit allocation at the macroblock layer under a constant frame rate assumption. The proposed rate control algorithm is able to adjust the encoding frame rate at the expense of tolerable time-delay. Furthermore, an R-D optimized hybrid DCT/wavelet scheme is used for effective I-frame coding. The new rate-control algorithm attempts to achieve a good balance between spatial quality and temporal quality to enhance the overall human perceptual quality at low bit rates. It is demonstrated that the rate control algorithm achieves higher coding efficiency at low bit rates with a low additional computational cost. The variable frame rate method and the hybrid I-frame coding scheme are compatible with the bitstream structure of H.263+. Second, we propose a new H.263+ rate control scheme which supports the variable bit rate (VBR) channel through the adjustment of the encoding frame rate and quantization parameter. A fast algorithm for the encoding frame rate control based on the inherent motion information within a sliding window in the underlying video is developed to efficiently pursue a good tradeoff between spatial and temporal quality. The proposed rate control algorithm also takes the time-varying bandwidth characteristic of the Internet into account and is able to accommodate the change accordingly. Finally, we present a region-based video coding scheme for video conferencing. It adopts a hybrid approach of block-based and object-based coding schemes. To achieve real-time processing, we consider a macroblock-based moving region segmentation by using image processing techniques, and propose an efficient rate control algorithm that keeps the spatial quality of each frame in a tolerable range without obvious motion unsmoothness under a channel of time-varying bandwidth. Furthermore, another rate control algorithm that improves the human visual perceptual quality for low VBR video is discussed and tested. These two rate control algorithms are implemented with the H.263 platform so that they are compatible with the existing H.263 standard.

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