 GOBS (Michel Bayard). An encoder that compresses using motion compensation presumes that successive frames of video will contain similarities. Thus, it models current frames after previous ones. The codec, working at the macroblock layer, compares a previously encoded frame (a reference frame) to an actual one. As it does so, it looks for motion vectors (a directional movement of groups of pixels). Upon seeing movement, the coder tells the decoder about which pixels are moving and their line of direction. Both codecs then assume this motion will continue. The coder transmits only the prediction error-motion that did not correspond to what was predicted. When the prediction error is minimal, it is not transmitted at all. When prediction error is transmitted, the decoder adds it to the previously coded image, and thereby creates a mathematical representation of the original block. The Video Communications: The Whole Picture encoder stores this and uses it to predict the next image. Were this done indefinitely, it could result in cumulative error. For that reason, one out of every 32 blocks is encoded using intraframe techniques, and thereby provides a fresh image. The requirement for an H.261 codec to decode motion vector information is mandatory, but an H.261-compliant codec does not have to send such information. Sending codecs that do use motion vectors often produce superior pictures at the receiving end, in comparison to sending codecs that do not. That is because they are generally three to four times more efficient (allowing better use of bandwidth). Block structure of an image Macro-block, consisting of four luminace and two chromance blocks Figure 7-5. Macroblocks (Michel Bayard). Forward Error Encoding The CCITT included forward error correction (FEC) as an option in H.261. Forward error correction can reduce the number of transmission errors in a video stream but it does SO at the expense of an increase bit rate. The H.261 standard allows a decoder to ignore an encoder's FEC codes, which provide little 