-T organization that oversees standards development for video. MPEG is relatively straightforward. In its initial compression step, MPEG creates what is called a "reference frame," an actual copy of the original frame of video. It intersperses these so-called I-Frames every 15 frames in the transmission. Because they are used as a reference point they are only minimally compressed. MPEG assumes (correctly) that a relatively small amount of the infor- mation in a series of video frames changes from frame to frame. The background, for example, stays relatively constant in most movies, for long periods of time. Therefore, only a small amount of the information in each frame needs to be recaptured, recompressed, and re-stored. Two additional frame types are created in the MPEG process. Pre- dicted frames use information contained in past frames to "predict" the 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. Premises Technologies Chapter 5 content of the next frame in the series, thus reducing the amount of information required. These predicted frames experience medium to sig- nificant compression. Finally, MPEG may create bidirectional interpolated frames that are interspersed between the original I-Frames and the predicted frames. They require input from both, and because they constitute the bulk of the transmitted frame stream, are subject to the most compression. In fact, compression ratios of 200:1 are not uncommon in MPEG. An 18 Gigabyte movie can be reduced to a somewhat more manageable 90 MB. There are other compression schemes for video. Motion JPEG, for example, is an intraframe compression technique (unlike MPEG, which is interframe) that compresses and stores each and every image as a sep- arate entity. This is different from MPEG. Compression ratios with motion JPEG are considerably lower than MPEG's 200:1 numbers; ratios of 20:1 ar