Open and closed gops – Apple Compressor 2 User Manual

Page 141

Chapter 10

Creating MPEG-2 Output Files

141

In principle, the less average motion there is from one frame to the next, the farther apart
P-frames can be spaced, and the greater the compression can be. For most video
material, the IBBP structure is a good choice. Material with unusually fast motion
throughout the entire sequence may benefit from an IBP or IP structure, but in such cases
a relatively high bit rate (6 to 8 Mbps for SD video) may be required for good quality.

GOP size
This setting specifies the number of frames within a GOP. Because exactly one I-frame
exists per GOP, longer GOP sizes generally provide greater compression, because B- and
P-frames are smaller than I-frames.

For most media, spacing I-frames about 1/2 second apart gives good results. This
equates to a GOP size of 15 frames for NTSC and 12 frames for PAL. The DVD-Video
specification prohibits GOP lengths from being much longer than this. Generally, only
material with scene changes occurring less than 1/2 second apart frequently
throughout the video will benefit from shorter GOP sizes.

Open and closed GOPs
Open GOPs are most efficient because they allow B-frames from one GOP to look at the
last P-frame from the preceding GOP as well as the first I-frame of their own GOP. Open
GOPs don’t have to start with I-frames as shown below:

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Open GOP

(IBBP, 15 frames)

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Closed GOP

(IBBP, 15 frames)