L64105 LSI Logic Corporation, L64105 Datasheet - Page 400

• Manufacturer Part Number: L64105
• Description: Mpeg-2 Audio/video Decoder
• Manufacturer: LSI Logic Corporation

A-6
decoding the bitstream. For example, a typical sequence of pictures in
display order might be as shown in
Figure A.2
In contrast, the bitstream order corresponding to the given display order
would be as shown in
Figure A.3
Because the B pictures depend on the subsequent I or P picture in the
display 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. Slices consist of a
header and one or more macroblocks . The slice header contains position
and quantizer scale information. A slice provides some immunity to data
errors. Should the bitstream become unreadable within a picture, the
decoder should be able to recover by waiting for the next slice without
having to drop an entire picture.
A macroblock is the basic unit for motion compensation and quantizer
scale changes. In MPEG-2, the macroblock can be either field or frame
coded. Each macroblock consists of a header and the six 8 x 8 blocks.
The macroblock header contains quantizer scale and motion
compensation information. A skipped macroblock is one for which no
DCT information is encoded.
Blocks are the basic coding unit, and the DCT is applied at the block
level. Each block is transformed into a set of frequency coefficients which
are quantized and encoded to reduce the number of bytes needed to
represent the block.
Video/Audio Compression and Decompression Concepts
0
0
I
I
B
P
1
3
B
B
2
1
P
B
3
2
Typical Sequence of Pictures in Display Order
Typical Sequence of Pictures in Bitstream Order
B
P
4
6
B
B
5
4
Figure
P
B
6
5
B
7
P
9
A.3.
B
8
B
7
P
B
9
8
Figure
10 11 12 13 14 15 16 17 18
12 10 11 15 13 14 18 16 17
B
I
A.2.
B
B
B
I
B
P
B
B
P
B
B
P
B
B
P
B