Autodesk 64006-051108-9001 User Guide - Page 109

MPEG 101 Group of Pictures The GOP (group of pictures) is a self-contained unit of MPEG frames, starting with an I-frame and typically ending on a B-frame. Altering the GOP size determines how frequently I-frames are added and is essentially the same as controlling the keyframe rate in a QuickTime movie. The standard MPEG GOP is 15 frames for NTSC (12 frames for PAL) with a P-frame distance of 3 and ending on a B-frame. This means a new I-frame is added every 1/2 second in 29.97 fps material (NTSC) or almost every 1/2 second in 25 fps (PAL). Increasing the I-frame rate by decreasing the size of the GOP has similar consequences to increasing the keyframe rate of a QuickTime movie. Depending on the material, an extremely small or large GOP may cause noticeable artifacting. I-frames, or intraframes, take the most bytes to encode because they contain the entire frame, and don't contain differences as with P-frames and B-frames. P-frames, or predictive frames, contain the changes between the current frame and the I-frame or P-frame that precedes it. This is similar to difference frames in QuickTime. B-frames, or bidirectional frames, can contain changes from the preceding/upcoming I-frames or P-frames. B-frames compress more than P-frames, but they also take longer to encode. The GOP can be either open or closed. An open GOP takes information from adjacent GOPs to improve quality and is best for MPEG-1 streams, but makes random access harder. A closed GOP only uses information from the frames within that GOP, which enables random access throughout the stream and is required for encoding DVD-Video. Using closed GOPs may produce lower-quality streams at the same bitrate, but the difference may not be discernible at higher bitrates. MPEG Licensing Depending on your use of MPEG, there may be licensing fees due to the companies which control the patents behind MPEG. For MPEG-2 use, the licensing agreement and fees are usually handled by MPEG LA, a company that has secured the necessary rights from the various MPEG-2 patent holders to license MPEG-2 to developers. For more information on MPEG-2 licensing, visit MPEG LA site at http://www.mpegla.com. The fact that you have purchased a copy of Cleaner does not grant a license to use MPEG-2 in a manner inconsistent with the MPEG LA Patent Portfolio License. MPEG-1 licensing is not as well organized and defined as MPEG-2 licensing, and it is somewhat unclear what your licensing responsibilities are when using it. You may find helpful information at http://www.mpeg.org or http://www.cselt.it/mpeg. MPEG Audio Layer-3 (MP3) audio also has licensing issues. See "MP3 Licensing" on page 97.