When the Federal Communications Commission (FCC) requested proposals for the HDTV standard, they received four proposals for digital HDTV from four consortia. After the evaluation phase, the FCC declined to pick a winner among the four, and instead suggested that all these consortia join forces and produce a single proposal. The resulting partnership has the exalted title of the “Grand Alliance.” Currently, the specifications for the digital HDTV system use MPEG-2 as the compression algorithm. The Grand Alliance system uses the main profile of the MPEG-2 standard implemented at the high level.
ITU – T Recommendation H . 2 6 3
The H.263 standard was developed to update the H.261 video conferencing standard with the experience acquired in the development of the MPEG and H.262 algorithms. The initial algorithm provided incremental improvement over H.261. After the development of the core algorithm, several optional updates were proposed, which significantly improved the compression performance. The standard with these optional components is sometimes referred to as H.263+ (or H.263+ +).
In the following sections we first describe the core algorithm and then describe some of the options. The standard focuses on noninterlaced video. The different picture formats addressed by the standard are shown in Table 18.8. The picture is divided into Groups of Blocks (GOBs) or slices. A Group of Blocks is a strip of pixels across the picture with a height that is a multiple of 16 lines. The number of multiples depends on the size of the picture, and the bottom-most GOB may have less than 16 lines. Each GOB is divided into macroblocks, which are defined as in the H.261 recommendation.
A block diagram of the baseline video coder is shown in Figure 18.16. It is very similar to Figure 18.10, the block diagram for the H.261 encoder. The only major difference is the ability to work with both predicted or P frames and intra or I frames. As in the case of H.261, the motion-compensated prediction is performed on a macroblock basis. The vertical and horizontal components of the motion vector are restricted to the range [−16, 15,5 ]. The transform used for representing the prediction errors in the case of the P frame and the pixels in the case of the I frames is the discrete cosine transform. The transform coefficients are quantized using uniform midtread quantizers. The DC coefficient of the intra block is quantized using a uniform quantizer with a step size of 8. There are 31 quantizers available for the quantization of all other coefficients with step sizes ranging from 2 to 62. Apart from the DC coefficient of the intra block, all coefficients in a macroblock are quantized using the same quantizer.
