X4

394 MPEG glossary
http://pro.magix.com
Field
A half-image, i.e. two halves which combine to produce a frame (see de-
interlacing (view page 395)).
Frame
A frame is a single image from a video sequence which also called a full image.
PAL video, for example, contains 25 frames per second, NTSC 29.97 frames.
Video recordings, with the exception of computer animations and still frames,
don't contain full images. Instead, they have double numbers of half-images
(fields) which are transmitted in an interlaced state. However, we still refer to
frames, since many predecessors of MPEG compression are based on such
frames. Video editing literature usually refers to frames.
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 397) and movement
approximation are methods used 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).