
L64005 MPEG-2 Audio/Video Decoder Technical Manual
1-3
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
Motion vectors give the displacement from the stored previous picture.
P-pictures use motion compensation to exploit temporal redundancy in
the video. Motion within the pictures means that the pixels in the previous
picture will be in a different position from the pixels in the current block,
and the displacement is given by motion vectors encoded in the MPEG
bitstream. Motion vectors dene the motion of a macroblock, which is the
motion of a 16 x 16 block of luminance pixels and the associated chromi-
nance components.
When an encoder provides B-pictures, it must reorder the picture
sequence so that the decoder operates properly. Because B-pictures use
motion compensation based on previously sent I- or P- pictures, they can
only be decoded after the referenced pictures have been sent.
As mentioned earlier, a macroblock is a 16 x 16 region of video, corre-
sponding to 16 horizontal pixels and 16 vertical display lines. When sam-
pling a block, the video encoder captures the luminance component of
every pixel in the horizontal direction, and the luminance component of
every line in the vertical direction. However, the encoder similarly cap-
tures only every other Cb and Cr chrominance component. The result is
a 16 x 16 block of luminance components and two 8 x 8 blocks each of
Cr and Cb components. Each macroblock then consists of a total of six
8 x 8 blocks (four 8 x 8 luminance blocks, one 8 x 8 Cr block, and one
Figure 1.1
MPEG Macroblock
Structure
It is important to note that the spatial picture area covered by the four
8 x 8 blocks of luminance is the same area covered by each of the 8 x 8
chrominance blocks. Because half as many chrominance samples are
needed to cover the same area, they t into an 8 x 8 block instead of a
16 x 16 block.
For a given macroblock, the encoder must choose a coding mode. The
coding mode depends on the picture type, the effectiveness of motion
8
88
YCr
Cb
01
23
45