MPEG-2 Video Coding - Digital Video and Audio Broadcasting Technology - page 118

possible as a result of this. The arrangement of frames occurring between
two I pictures, i.e. complete pictures, is called a group of pictures (GOP)
(Fig.7.10.).
The motion estimation for obtaining the motion vectors proceeds as fol-
lows: Starting with a delta frame to be encoded, the system looks in the
preceding frame (forward prediction P) and possibly also in the subsequent
frame (bidirectional prediction B) for suitable macroblock information in
the environment of the macroblock to be encoded. This is done by using
the principle of block matching within a certain search area around the
macroblock.

Matching window

Frame N1,
motion vector
forward

Frame N+1,
motion vector
backward

Frame N,
B encoded
macro block

Fig. 7.12. Motion vectors

If a matching block is found in front, and also behind in the case of bi-
directional coding, the motion vectors are determined forward and back-
ward and transmitted. In addition, any additional block delta which may be
necessary can also be transmitted, both forward and backward. However,
the block delta is coded separately by DCT with quantization, described in
the next chapter, a method which saves a particularly large amount of stor-
age space.
A group of pictures (GOP) then consists of a particular number and a
particular structure of B pictures and P pictures arranged between two I
pictures. A GOP usually has a length of about 12 frames and corresponds
to the order of I, B, B, P, B, B, P, .... The B pictures are thus embedded be-
tween I and P pictures. Before it is possible to decode a B picture at the re-