4, pictures are coded as one luminance and two color difference components (Y, Cb, and Cr). Reference should be made to CCIR Rec. 601 (Ref. 23) for their components and codes representing their sampled values. For example: Black = 16 White = 235 Zero color difference = 128 Peak color difference = 16 and 240 The values given are nominal values and the coding algorithm functions with input values of 1 through 254. Two picture scanning formats have been specified. For the first format (CIF), the luminance structure is 352 pels per line, 288 lines per picture in an orthogonal arrangement. The color-difference components are sampled at 176 pels per line, 144 lines per picture, orthogonal. Figure 16.10 shows the color- difference samples being sited such that the block boundaries coincide with luminance block boundaries. The picture area covered by these numbers of pels and lines has an aspect ratio of 4:3 and corresponds to the active portion of the local standard video input. It should be noted that the number of pels per line is compatible with sampling the active portions of the luminance and color-difference signals from 525- or 625-line sources 10 FEC coding. Consult Ref. 21 for a discussion of FEC. 16.10 CONFERENCE TELEVISION Luminance sample Chrominance sample Block edge Figure 16.10 Positioning of luminance and chrominance samples. [From Figure 2/H.261, ITU-T Rec. H.261 (Ref. 20).] at 6.75 MHz or 3.375 MHz, respectively. These frequencies have a simple relationship with those in ITU-R Rec. BT-601 (Ref. 19). The second format, called quarter-CIF or QCIF, has half the number of pels and half the number of lines stated of the CIF format. All codecs must be able to operate using QCIF. A means is provided to restrict the maximum picture rate of encoders by having at least 0, 1, 2, or 3 nontransmitted pictures. Both CIF/QCIF and minimum number of nontransmitted frames option is selectable externally. Video Source Coding Algorithm. A block diagram of the coder is illustrated Figure 16.11. The principal functions are 