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4:2:0
DVD takes it to another level by using a modified 4:2:2 sampling struc-
ture called 4:2:0. 4:2:0 samples R-Y half as much as Y and skips B-Y
st
on the 1
line. However, on the next horizontal line, B-Y is sampled half
as much as Y and R-Y is skipped. This routine is repeated effectively
reducing the color components by another half achieving ½ compres-
sion or a 125Mbit/sec bandwidth.
Through interpolation, the 4:2:0 is reconstructed into 4:2:2 without the
extra bandwidth requirement.
ONE
Samples taken in the Horizontal direction up to 720 per Line
PIXEL
LINE 1
Y
R-Y
Y
B-Y
LINE 2
Y
R-Y
LINE 3
Y
B-Y
LINE 4
.
.
.
.
LINE 480
Non Sampled components Illustrated as clear boxes
Discrete Cosine Transform
After the Selective Sampling process is complete, the next step in the
MPEG process is to remove very fine picture detail imperceptible to the
human eye. It is imperceptible because it is typically masked by other
picture content.
In a Video Frame, the very fine picture details consist of high frequency
information which are basically fast changing Luminance and Color
content. In contrast, low frequency information are slow changing
Luminance and Color content.
Y
Y
R-Y
Y
Y
B-Y
Y
Y
R-Y
Y
Y
B-Y
11
The high frequency information consumes the most data real estate and
is where we focus to compress in this next stage.
The process of eliminating the imperceptible information is called Dis-
crete Cosine Transform or DCT. The sampling process converts the
information into digital data as described previously. Each digital picture
frame is then sectioned off into 5400 blocks each consisting of 8 pixels
wide X 8 pixels high.
Y
Y
Y
Y
DCT transforms the 8X8 group of pixel values into frequency compo-
nents.
Although pixel values vary randomly in the 8X8 block, DCT re-positions
low frequency components on the upper left corner and high frequency
components on the lower right of the block.
Through an additional numerical conversion process called "Quantiza-
tion", frequency component values are assigned.
High freq components are identified by the zero values (lower right) and
low freq components by the larger values (upper left).
1 of 5400
Blocks
Picture
Frame
8 X 8
Pixel
Block
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