(b) Video Decoder Control Parameters Coding Controller To Video Variable Length Multiplex Coder Quantizer Coder Video (VLC) Input Inverse Quantizer Intraframe Up Interframe Down Motion Loop Filter Estimation and Compensation Motion Vectors (c) H.261 Source Encoder Implementation Figure 7.4-8 ITU-T H.261 block diagrams. H.261 specifies a set of encoder protocols and decoder encoding modes. It is made up of four adjacent 8 X 8 operations that every compatible system must follow. luminance blocks and two subsampled 8 X 8 color dif- The H.261 video multiplex defines the data structure ference blocks (Cb and Cr as defined in Table 7.4-4) hierarchy that the decoder can interpret unambiguously. corresponding to the luminance blocks. The GOB is The video data hierarchy defined in H.261 is depicted made up of 176 X 48 pixels (33 macroblocks) and is used in Fig. 7.4-9. They are the picture layer, group of to construct the 352 X 288 pixel CIF or 176 X 144 pixel block (GOB) layer, macroblock (MB) layer, and the basic QCIF picture layer. (8 X 8) block layer. Each layer is built from the previous The headers for the GOB and picture layers contain or lower layers, and contains its associated data payload, start codes SO that the decoder can re-synchronize when and header that describes the parameters used to gen- errors occur. They also contain other relevant in- erate the bit stream. The basic 8 X 8 block is used in formation required to reconstruct the image sequence. intraframe DCT encoding. The macroblock is the The following parameters used in the headers of the data smallest unit for selecting intraframe or interframe hierarchy complete the H.261 video multiplex. Video compression CHAPTER 7.4 Group of Block GOB (CIF) Group of Block GOB (ACIF) GOB 1 GOB 2 GOB 1 GOB 3 GOB 5 GOB 3 MB 11 GOB 10 GOB 11 GOB 12 MB 23 MB 24 MB 33 Macroblocks MB Basic Block Structure Macroblock Structure Figure 7.4-9 H.261 block hierarchy. Picture layer Quantizer (MQUANT), 5-bit normalized quantizer step size from 1-31. Picture start code (PSC), 2