Apple Compressor 2 - Manual (Page 140)

140 Chapter 10 Creating MPEG-2 Output Files
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Things to Consider When Choosing a GOP Setting
You need to consider the following aspects 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.
Understanding GOPs and Frame Types
A major feature of MPEG-2 encoding is its ability to remove redundancy, not only
within a frame, but also among a group of frames. MPEG-2 uses three frame types
(I, P, and B) to represent the video. A group of pictures (GOP) setting defines the
pattern of the three frame types used. These three picture types are defined in the
following way:
 Intra (I): Also known as the keyframe. Every GOP contains one I-frame. The I-frame is
the only MPEG-2 frame type which can be fully decompressed without any
reference to frames that precede or follow it. It is also the most data-heavy,
requiring the most disk space. If you want to place an I-frame at a scene change or
some other specific frame location, you need to manually set it using the Preview
window. This is known as a forced I-frame. See “
Adding Markers to a Clip ” on
page 220 for more information.
 Predicted (P): Encoded from a “predicted” picture based on the closest preceding I-
or P-frame. P-frames typically require much less disk space than do I-frames
because they reference a preceding I- or P- frame in the GOP.
Note: Both I-frames and P-frames are also known as reference frames, because a
B-frame may refer to either one or both frame types.
 Bi-directional (B): Encoded from an interpolation of succeeding and preceding
reference frames, either I-frame or P-frame. B-frames are the most storage-efficient
MPEG-2 frame type, requiring the least amount of disk space.
The use of B- and P-frames is what allows MPEG-2 to remove temporal redundancy,
contributing to its ability to compress video efficiently.