 of a video sequence occur because objects move. By focusing on the motion that has occurred between frames, motion compensation coding significantly reduces the amount of data that must be transmitted. Motion compensation coding compares each frame with the preceding frame to deter- mine the motion that has occurred between the two frames. It compensates for this motion by describing the magnitude and direction of an object's movement (e.g., a head moving right). Rather than completely regenerating any object that moves, motion compensation coding simply commands that the existing object be moved to a new location. Once the motion compensation techniques estimate and compensate for the motion that takes place from frame to frame, the differences between frames are smaller. As a TELEVISION TRANSMISSION result, there is less image information to be transmitted. Intraframe coding techniques are applied to this remaining image information. 16.9.4 An Overview of the MPEG-2 Compression Technique This section is based on the ATSC (Advanced Television System Committee) version of MPEG-2, which is used primarily for terrestrial broadcasting and cable TV. The objective of the ATSC standard (Refs. 16, 17) is to specify a system for the transmission of high-quality video, audio, and ancillary services over a single 6-MHz channel. The ATSC system delivers 19 Mbps of throughput on a 6-MHz broadcasting channel and 38 Mbps on a 6-MHz CATV channel. The video source, which is encoded, can have a resolution as much as five times better than conventional NTSC television. This means that a bit rate reduction factor of 50 or higher is required. To do this the system must be efficient in utilizing the channel capacity by exploiting complex video and audio reduction technology. The objective is to represent the video, audio, and data sources with as few bits as possible while preserving the level of quality required for a given application. A block diagram of the ATSC system is shown in Figure 16.7. This system model consists of three 