Apple Compressor 3 - Manual (Page 211)

Compressor Native Projects
You can set chapter markers using the Compressor Preview window, but they will not
be preserved when exported to versions earlier than DVD Studio Pro 2. I-frames will be
preserved. If you are using a version prior to DVD Studio Pro 2, you can manually set the
chapter markers directly in DVD Studio Pro.
Things to Consider When Choosing a GOP Setting
You need to consider the following factors when choosing a GOP setting.
GOP Structure
This setting specifies whether there will be two, one, or no B-frames between the reference
frames within a GOP. GOP structure, along with GOP size, determines the number of I-,
P-, and B-frames that will be used during transcoding.
The GOP structure you choose depends on how far apart P-frames should be spaced.
Since a P-frame is predicted from the previous reference frame (either an I-frame or a
P-frame), if there are one or two B-frames in between, the prediction must cover the
distance objects can move over the duration of two to three frames.
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.
211 Chapter 18 Creating MPEG-2 Output Files