videoconference. The first stage tor'. The overhead of transmitting a vector for individual pels would of bit rate reduction is achieved by reducing the sampling rates to more than offset the savings elsewhere so the technique is applied below those required by CCIR 601. CCITT sought an international to blocks of pels. In reality, motion is rarely purely translational but standard for world-wide application which would permit users to includes rotation and deformation and the edges of moving objects converse without having to concern themselves over the existence rarely coincide exactly with block boundaries. Inter-frame dif- of the two different television scanning standards, namely 625/50 ferences also result from uncovered background areas and from and 525/60. illumination variations. Nevertheless motion compensation sub- To meet this goal, the coding kernel operates on a single format stantially reduces the energy of inter-frame differences and can known as the Common Intermediate Format (CIF) or its simply yield a halving of bit consumption. related, downsized version called Quarter CIF (QCIF). The lumin- Because the motion compensation is applied to pictures in their ance component of CIF has 288 lines of 352 coded pels, repeated pel domain representation, it cannot be inserted directly in the 29.97 times per second. The two colour components each have half scheme of Figure 57.27. Instead the DCT is positioned inside the these numbers of pels and lines. QCIF has half the corresponding loop along with an inverse transform (IDCT) as shown in Figure spatial sampling densities of CIF and is more suitable for the lower 57.28 to give a motion compensated hybrid DPCM/DCT codec. bit rates especially when the decoded images are for display on This generic structure has attracted a great deal of attention by small screens. CIF and QCIF are a technical compromise between standardisation bodies and is the basis of the algorithms from 625/50 and 525/60, the number of lines being simply related to the CCITT, ISO and CCIR