) - b(i-d1,j-d2.k-1) - other words, no basic block encoding is used when the block content does not change significantly. The CPB (7.4.13) parameter encodes 63 combinations of the four lumi- nance blocks and two color difference blocks using where b is a 16 X 16 macroblock, i and j are its spatial a variable length code. The conditions for using CBP are pixel indices, k is the frame index, and di and d2 are the not specified in the H.261 recommendation. pixel displacements of the macroblock in the previous Motion compensated blocks can be chosen to be low frame. The displacements range from -15 < d1, d2 < + pass filtered before the prediction error is generated by 15. When d1 and d2 are set to zero, the DMD becomes the feedback loop. This mode is denoted as Inter + MC the macroblock difference (MD). The compression mode determines the operational encoder elements that are used for the current frame. The H.261 compression modes are depicted in Table 7.4-5. Table 7.4-5 H.261 Macroblock video compression modes. 3. The video multiplex coder processes each macroblock to generate the H.261 video bitstream whose ele- MQUANT TCOEFF ments are discussed above. Intra There are five basic MTYPE encoding mode decisions Intra that are carried out in step 2. These are, Inter Use intraframe or interframe mode? Inter Use motion compensation? Use a coded block pattern (CBP)? Inter + MC Use loop-filtering? Inter + MC Change quantization step size MQUANT? Inter + MC To select the macroblock compression mode, the vari- Inter + MC + LF ances (VAR) of the input macroblock, the macroblock Difference (MD) and the displaced macroblock differ- Inter + MC + LF ence (DMD) (as determined by the best motion esti- Inter + MC + LF mate) are compared as follows, Video compression CHAPTER 7.4 + LF in Table 7.4-5. The low pass filter is intended to REC_LEVEL=QUANTx(2xCOEFF_VALUE-1), = reduce the quantization noise in the feedback loop, as for COEFF_LEVEL well as the high frequency noise and artifacts introduced by the motion compensator. H.261 defi