ps channels that are available for transmission. Figure 7-3. YCbCr 4:1:1 Format (Michel Bayard). H.261 Options As we have stated, H.261 is a permissive standard. It allows for a great deal of flexibility. This flexibility can confuse video communication system buyers (and, for that matter, video communication system vendors). As we mentioned above, QCIF, a lower-resolution picture format, is mandatory under H.261 and CIF is optional. CIF and QCIF can code at varying frame rates: 7.5, 10, 15, or 30 per second. As we said in the previous chapter, dropping frames is common as a means of compressing data for transmission. A codec that produces only 7.5 fps is, technically speaking, just as "H.261 compliant" as a codec that produces 30 fps. This vagueness in the standard allows a manufacturer to slow the frame rate to deliver CIF resolutions (which come at the expense of motion handling). On the other hand, no matter how many frames are sent in a given second, a receiving Standards for Videoconferencing codec must decode up to and including 30 frames per second. Therefore, an evaluator can only assess the result of a coder's capacity by viewing the decoder. Stated another way, when one compares interoperating codecs, what one sees on the near end is the result of the work of the codec at the far end. Motion Compensation As we just said, H.261 specifies, as an option, motion compensation. Motion compensation is performed in addition to intraframe transform (DCT) encoding, which is mandatory. While intraframe coding removes an image's spatial redundancy, interframe encoding works on temporal (frame-to-frame) redundancy. In all cases, H.261 encoding will start out with an intraframe-coded image that has been compressed using a combination of DCT, Huffman and run-length encoding. It must do this in order to have an original frame of video that can become the basis for the motion compensation process. Block Structure of an Image Group of blocks, consisting of 33 macro-blocks Figure 7-4. Subdivision Of A Frame Of Video Into