mpatibility whenever possible. Compressing Moving Images Growth in videoconferencing, on-demand training, and gaming is fueling the growth in digital video technology but the problems mentioned pre- viously still loom large. Recent advances have had an impact; for exam- ple, storage and transport limitations can often be overcome with compression. The most widely used compression standard for video is MPEG, created by the Moving Pictures Expert Group, the joint ISO/IEC/ITU-T organization that oversees standards development for video. MPEG is relatively straightforward. There are three types of MPEG frames created during the compression sequence. They are intra (I) frames, Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Chapter 2 predicted (P) frames, and bidirectional (B) frames. An intraframe or I frame is nothing more than a frame that is coded as a still image and used as a reference frame. Predicted frames, on the other hand, are pre- dicted from the most recently reconstructed I or P frame. B frames are predicted from the closest two I or P frames, one from the past and one in the future. Imagine the following scenario: You are converting a piece of video that you shot at the beach to MPEG. The scene, shown in Figure 2-22, lasts 6 seconds, and is nothing more than footage of the fishing boat moving slowly in front of the camera, which is locked down on a tripod. Remember that video captures a series of still frames, one every 1/30 of a second (30 frames per second). What MPEG does, in effect, is an analysis of the video based on the reference (I) frames, the predicted frames, and the bidirectional frames. From the image shown in the illustration, it should be clear that very little changes from one frame to another in a 1/30-second interval. The boat may move slightly (but very slightly), and the foam that the propeller is chu