analog-to-digital (A/D) conversion of color video signals. An H.261 source coder manipulates non-interlaced pictures in accordance with a portion of that Recommendation. To understand H.261, one must possess some knowledge of CCIR-601's characteristics. CCIR-601 PAL/SECAM Characteristics 525/60 625/60 Scanning Interlaced Interlaced Technique Interlaced Interlaced Fields sampled per second Vertical and Vertical and sampling Horizontal Horizontal Horizontal Horizontal direction Luminance 720 H X 485 V 720 H X 576 V 352 H X 288 V 176 H X 144 V resolution Chrominanc 360 H X 485 V 360 H X 576 V 176 H X 144 V 88 H X 72 V e resolution Chroma YCbCr 4:2:2 YCbCr 4:2:2 YCbCr 4:1:1 YCbCr 4:1:1 sampling Figure 7-1. CCIR-601 Picture Formats. CCIR-601 addresses the incompatibility of the world's three television formats (NTSC, PAL and SECAM) by defining two picture structures that will work with any of them. These are the Common Intermediate Format (CIF) and the Quarter Common Intermediate Format (QCIF). In Figure 7-1, the four CCIR-601 picture structures are presented, along with the techniques used to derive them. Since H.261 is concerned only with CIF and QCIF resolutions, in our discussion of CCIR-601, we will address only CIF and QCIF image formats. Standards for Videoconferencing Both CIF and QCIF are compromises that use the frame rate of North American TV (30 fps), and a resolution that easily adapts to that of European television. Compliance with CIF (also called full-CIF or FCIF) is optional under H.261, while compliance with QCIF is mandatory. At one-quarter the spatial resolution of CIF, QCIF is best suited to small screen (20" and below) and "talking head" (PC window) video communication applications. CIF is much more important to large screen (group system) applications. The CCIR-601 A/D conversion process begins with frequency filtering. A loop filter separates the frequencies contained in non-interlaced CIF or QCIF images into horizontal and vertical functions. The output of the filter is a two-dimensional array