Rate-Distortion Optimization for Video Compression

Motion video data consists essentially of a time-ordered sequence of pictures, and cameras typically generate approximately 24, 25, or 30 pictures (or frames) per second. This results in a large amount of data, and therefore demands the use of compression. For example assume that each picture has a relatively low \QCIF" resolution, i.e., 176 144 samples, that each sample is digitally represented with 8 bits, and that we skip two out of every three pictures in order to cut down the bit rate. For color pictures, three color component samples are necessary to represent a su cient color space for each pixel. In order to transmit even this relatively lowdelity sequence of pictures, the raw source data rate is still more than 6 Mbit/s. However, today's low-cost transmission channels often operate at much lower data rates so that the data rate of the video signal needs to be further compressed. For instance, using V.34 modems that transmit at most 33.4 Kbit/s over dial-up analog phone lines, we still need to compress the video bit rate further by a factor of about 200 (more if audio is consuming 6 Kbit/s of that same channel or if the phone line is too noisy for achieving the full bit rate of V.34).