Apple Final Cut Pro 7 - Manual (Page 1911)

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.
Open and Closed GOPs
An open GOP allows the B-frames from one GOP to refer to an I- or P-frame in an adjacent
GOP. Open GOPs are very efficient but cannot be used for features such as multiplexed
multi-angle DVD video. A closed GOP format uses only self-contained GOPs that do not
rely on frames outside the GOP.
1911 Appendix B Video Formats