
1-6
Introduction
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
A GOP is a random access point; that is, it is the smallest coding unit
that can be independently decoded within a sequence, and consists of a
header and some number of pictures. The GOP header contains time
and editing information.
The three types of pictures as explained earlier are:
I-pictures
P-pictures
B-pictures
Note that because of the picture dependencies, the bitstream order (the
order in which pictures are transmitted, stored, or retrieved), is not the
display order, but rather the order in which the decoder requires the pic-
tures for decoding the bitstream. For example, a typical sequence of pic-
tures, in display order, might be as shown in
Figure 1.2.Figure 1.2
Typical Sequence
of Frames in
Display Order
In contrast, the bitstream order corresponding to the given display order
Figure 1.3
Typical Sequence
of Frames in
Bitstream Order
Because the B-pictures depend on the subsequent I- or P-picture in dis-
play order, the I- or P-picture must be transmitted and decoded before
the dependent B-pictures.
Pictures consist of a header and one or more
slices. The picture header
contains time, picture type, and coding information.
A slice provides some immunity to data errors. Should the bitstream
become unreadable within a picture, the decoder should be able to
I
B
P
B
P
B
P
B
I
B
P
B
P
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
I
P
B
P
B
P
B
I
B
P
B
P
B
0
3
1
2
6
4
5
9
7
8
12
10
11
15
13
14
18
16
17