Michel Bayard). Layered Block Encoding An H.261 encoder multiplexes the bits it passes to a HRD by breaking a picture into four hierarchical layers. A picture (a frame of video) represents the highest layer. This picture has been subdivided into groups of blocks (GOB). Each GOB is sub-divided into macroblocks and each macroblock contains 8-by-eight pixel blocks. Sub-dividing an image into successively smaller blocks permits orderly coding and orderly transmission. Start codes define the two highest layers: a picture is indicated by a 20- bit start code; a GOB is declared by a 16-bit code. At the macroblock layer and below, codewords are used. A macroblock uses a variable length codeword to indicate where it sits within a GOB. A pixel block uses a codeword to indicate which coefficient is to be used during reverse-transform At the end of each pixel block there is also an end-of-block marker. A frame of video is broken into GOBs, each of which contains a specific number of luminance and color-differenced pixels. In a GOB, luminance is conveyed using 176 vertical pixels and 48 horizontal pixels. The pixels that convey color difference (Cb Video Communications: The Whole Picture and Cr) are spatially arranged with reference to luminance. There are more GOBs in a CIF-resolved image than in one that is QCIF-resolved. The higher-resolution CIF picture requires 12 GOBs; QCIF needs only three. A GOB is subdivided in to 33 macroblocks. A macroblock represents luminance using 16-by-16 pixel blocks (four 8x8 blocks). The four luminance blocks are overlaid with two color-differenced (CR and CB) chrominance blocks, which cover the same area as the luminance blocks but at half their resolution. At the bottom of the hierarchy is the block layer. A block is comprised of eight-by- eight pixels. These pixel blocks are encoded using the DCT and, possibly, using motion compensation (predictive encoding). We have taken the reader step-by-step through the coding specification laid out in the ITU-T's H.261 specification. We conclude our