Apple Final Cut Express 4 User Manual
Page 1076
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GOP Length
Longer GOP lengths encode video more efficiently by reducing the number of I-frames
but are less desirable during short-duration effects such as fast transitions or quick
camera pans. MPEG video may be classified as long-GOP or short-GOP. The term
long-GOP refers to the fact that several P- and B-frames are used between I-frame
intervals. At the other end of the spectrum, short-GOP MPEG is synonymous with
I-frame–only MPEG. Formats such as IMX use I-frame–only MPEG-2, which reduces
temporal artifacts and improves editing performance. However, I-frame–only formats
have a significantly higher data rate because each frame must store enough data to be
completely self-contained. Therefore, although the decoding demands on your
computer are decreased, there is a greater demand for scratch disk speed and capacity.
Maximum GOP length depends on the specifications of the playback device. The
minimum GOP length depends on the GOP pattern. For example, an IP pattern can
have a length as short as two frames.
Here are several examples of GOP length used in common MPEG formats:
 MPEG-2 for DVD: Maximum GOP length is 18 frames for NTSC or 15 frames for PAL.
These GOP lengths can be doubled for progressive footage.
 1080-line HDV: Uses a long-GOP structure that is 15 frames in length.
 720-line HDV: Uses a six-frame GOP structure.
 IMX: Uses only I-frames.