0-bit synchronization pat- tern (0000 0000 0000 0001 0000). Motion vector data (MVD), up to 11-bit VLC describing the differential displacement. Temporal reference (TR), 5-bit input frame number. Type information (PTYPE), indicates source format, Coded block pattern (CBP), up to 9-bit VLC indicat- CIF=I QCIF=0, and other controls. ing the location of the encoded blocks in the macroblock. User-inserted bits. GOB layer Block layer Group of blocks start code (GBSC), 16-bit synchroni- Transform coefficients (TCOEFF), are zigzag scanned zation code (0000 0000 0000 0001). and can be 8-bits fixed or up to 13-bit VLC. Group number (GN), 4-bit address representing the End of block (EOB), symbol. 12 GOBs per CIF frame. The H.261 bit stream also specifies transmission syn- Quantizer information (GQUANT), indicates one of chronization and error code correction using a BCH code 31 quantizer step sizes to be used in a GOB unless [20] that is capable of correcting 2-bit errors in every overridden by macroblock MQUANT parameter. 511-bit block. It inserts 18 parity bits for every 493 data User-inserted bits. bits. A synchronization bit is added to every codeword to be able to detect the BCH codeword boundaries. The transmission synchronization and encoding also operates Macroblock layer on the audio and control information specified by the Macroblock address (MBA), is the position of a macro- ITU H.320 Recommendation. block within a GOB. The H.261 video compression algorithm depicted in Type information (MTYPE), for one of 10 encoding Fig. 7.4-8 (c) is specified to operate in intraframe and modes used for the macroblock. This includes interframe encoding modes. The intraframe mode pro- permutations of intraframe, interframe, motion vides spatial encoding of the 8 X 8 block, and uses the compensation (MC), and loop filtering (LF). A two-dimensional DCT. Interframe mode encodes the pre-specified VLC is used to encode these modes. prediction error, with motion compensation being CHAPTER 7.4 Video compression optional. The prediction