prediction, block transformation, and quantization. The picture error (INTER mode) or the input picture (INTRA mode) is subdivided into 8-pel-by-8-pel line blocks, which are segmented as transmitted or nontransmitted. Furthermore, four luminance blocks and two spatially corresponding color-difference blocks are combined to form a macroblock. Transmitted blocks are transformed, and the resulting coefficients are quantized and then variable length coded. Motion compensation is optional. The decoder will accept one vector per macroblock. The components, both horizontal and vertical, of these motion vectors have integer values not exceeding +15. The vector is used for all four luminance blocks in the macroblock. The motion vector for both color-difference blocks is derived by halving the component values of the macroblock vector and truncating the magnitude parts toward zero to yield integer components. A positive value of the horizontal or vertical component of the motion vector signifies that the prediction is formed from pels in the previous picture, which are spatially to the right or below the pels being predicted. Motion vectors are restricted such that all pels referenced by them are within the coded picture area. Loop Filter. A two-dimensional spatial filter may be used in the prediction process. The filter operates on pels within a predicted 8-by-8 block. It is separable into one-dimensional horizontal and vertical functions. Both are nonrecursive carrying coefficients of 111 4.2.4 TELEVISION TRANSMISSION Video To video mutiplex coder Transform Quantizer Picture memory with motion compensated variable decay Loop filter CC Coding control Flag for INTRA/INTER Flag for transmitted or not Quantizer indication Quantizing index for transform coefficients Motion vector Switching on/off of the loop filter Figure 16.11 Functional block diagram of the source coder. [From Figure 3/H.261, ITU-T Rec. H.261 (Ref. 20).] except at block edges, where one of the taps would fall outside the block. In this case the one-dimensio