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 ways.
• Intra (I): Also known as the key frame. 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
Working with Markers and Poster Frames
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.