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4:1:0 This is the color coding used for DVDs and most other consumer video
applications. For each 4 pixels grouped together on two rows, one unit of color
information is saved. This corresponds to a output data compression of 1/2.
Field
A half-image, i.e. two halves which combine to produce a frame (see de-interlacing
(view page 373)).
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 374) 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).