 of values. Block structure of an image Whole image, consisting of 12 or 3 groups of blocks Figure 7-2. CIF and QCIF (Michel Bayard). The values that result from frequency filtering are of two basic types: luminance and chrominance. The human visual system is more perceptive to subtle changes in light than to subtle changes in color. For that reason, the CCIR-601 conversion process allots higher spatial resolution for luminance than for chrominance, specifically for every four luma (Y) samples, two color-differenced samples (Cb and Cr) are taken. This method, known as chroma subsampling, results in a format known as YCbCr 4:1:1. As we discussed in Chapter 6, quantization follows sampling. Once scanned, the uncoded YCbCr data is quantized according to CCIR-601. Luminance (grayscale) information is quantized using one of 220 different levels. Color-difference information is quantized as one of 255 different levels. These quantization levels are the same as those used in the DCT (which, in H.261 encoding, is the next step after quantizing). Video Communications: The Whole Picture One of two different types of compression is applied to the 8-by-8 pixel blocks: intraframe and interframe. It is possible to perform both intraframe and interframe coding on a pixel block but only intraframe coding is mandatory in H.261. Intraframe encoding relies on the DCT (see Chapter 6). Interframe encoding uses DPCM, and motion compensation techniques (also described in Chapter 6). H.261 codecs that perform both DCT and motion compensation are hybrid coders. Of course, H.261 codecs operate in pairs or multiples. Each has a coder and a decoder component. The coder on one end talks to the decoder on the other. Between them flows a bit stream that is arranged into 512-bit frames. Each frame contains two synchronization bits, 492 data bits that describe the video frame, and 18 error- correction code bits. These 512 bits are then interlaced with the audio stream and transmitted over Px64 channels in which 'P' represents the number of 64 Kb