
A-6
Video/Audio Compression and Decompression Concepts
Figure A.2
Typical Sequence of Pictures in Display Order
In contrast, the bitstream order corresponding to the given display order
Figure A.3
Typical Sequence of Pictures in Bitstream Order
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 eld 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 coefcients which
are quantized and encoded to reduce the number of bytes needed to
represent the block.
A.1.3 Video Decoding
Video decoding is the reverse of video encoding and is intended to
reconstruct a moving picture sequence from a compressed, encoded
bitstream. Decoding is simpler than encoding because there is no motion
estimation performed and there are far fewer options.
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