X5

MPEG glossary 359
GOP
Group of Pictures: The sequence of I frames and the P and B frames that belong to
them.
e.g. I B B P B B P B B I ...
(This GOP has a length of 9, with 2 P frames and 2 B frames)
I frames contain the entire image information of a frame, while P and B have part of
the information. So-called prediction (view page 361) and movement approximation
are methods u
sed for reduction.
The combination P B B is called a subgroup.
I frames must appear in regular intervals in the data stream for image and sound to be
synchronized. Between the I frames only a limited count of P and B frames is allowed.
This explains a few things: Since P and B frames contain only differential information,
these differences will be larger with time, since more and more changes takes place
from frame to frame. A large count does not make much sense, since GOP has a
maximum length of 15 (4P, 2B) in PAL and 18 (5P, 2B) in NTSC. (More than 2 B
frames between P frames is not allowed).
In a closed GOP, B frames of the last subgroup may contain only backward
predictions or references to the preceding P frame, but no references to the following I
frame, since it belongs to the next GOP.
I frames
Intra-frames: In these pictures, the entire image information of a frame is saved and
only information from this frame is used ("intra-frame encoded"). In contrast to the I
frame, P and B frames save only the differences between the current frame, and
preceding and/or following frame are also found in MPEG video (P frame = "predicted
frame", B frame = "bidirectional frame", see Prediction (view page 361)).
Interlace
For historical reasons, pictures in a movie are always recorded and transmitted in the
form of two fields, first the lines with even numbers and then those with odd numbers.
These fields are alternatively displayed with a double-frame rate. The (lazy) eye of the
viewer or the processing of the TV tube puts the two frames together to form one.