ign of the rate control algorithm 7.4.7 Closing remarks is not specified. The DCT macroblock coefficients are subjected to variable thresholding before quantization. The thresh- Digital video compression, although only recently be- old is designed to increase the number of zero valued coming an internationally standardized technology, is coefficients, which in turn increases the number of the strongly based upon the information coding technologies zero run-lengths and VLC coding efficiency. The ITU- researched over the last 40 years. The large variety of T Reference Model 8 provides an example thresholding bandwidth and video quality requirements for the algorithm for the H.261 encoder. Nearly uniform transmission and storage of digital video information has scalar quantization using a dead zone is applied after demanded that a variety of video compression tech- the thresholding process. All the coefficients in the niques and standards be developed. The major in- luminance and chrominance Macroblocks are subjected ternational standards recommended by ISO and the ITU to the same quantizer except for the intraframe DC make use of common video coding methods. The gen- coefficient. The intraframe DC coefficient is quantized eralized digital video encoder introduced in Section using a uniform scalar quantizer whose step size is 8. 7.4.2, illustrates the spatial and temporal video com- The quantizer decision levels are not specified, but the pression elements that are central to the current MPEG- reconstruction levels are defined in H.261 as follows, 1, MPEG-2/H.262, MPEG-4, H.261, and H.263 stan- For QUANT odd dards that have been developed over the past decade. They address a vast landscape of application re- quirements, from low to high bitrate environments, as REC_LEVEL = QUANT (2 <COEFF_VALUE+1) well as stored video and multimedia to real-time video- for COEFF_LEVEL>0. conferencing and high quality broadcast television. CHAPTER 7.4 Video compression References 1. ISO/IEC 11172 Information 9. D. Huffman, "A method 