MPEG-2 Video Coding - Digital Video and Audio Broadcasting Technology - page 112

sound signals but these must be transmitted coded separately according to
MPEG. The horizontal and vertical blanking intervals are, therefore, left
out completely in MPEG. The horizontal and vertical blanking intervals
and all signals in them can be regenerated again without problems at the
receiving end.
A PAL signal has 625 lines, only 575 of which are visible. The differ-
ence of 50 lines, divided by 625, is 8 % which is the saving in data rate
achieved when the vertical blanking is omitted. The length of one line is
64 µs but the active video area is only 52 µs which, divided by 64,
amounts to a further saving of 19 % in the data rate. Since there is some
overlap in the two savings, the total result of this redundancy reduction is
about 25 %.

7.1.3 Reduction in Vertical Color Resolution (4:2:0)

The two color difference signals C B and C R are sampled at half the data
rate compared with the luminance signal Y. In addition, the bandwidth of
C B and C R is also reduced to 2.75 MHz in comparison with the luminance
bandwidth of 5.75 MHz - a 4:2:2 signal (Fig. 7.3.). However, the color
resolution of this 4:2:2 signal is only reduced in the horizontal direction.
The vertical color resolution corresponds to the full resolution resulting
from the number of lines in a television frame.

Column m Column m + 1

Column m Column m + 1 Column m + 2

Cb

YCb YCb

Y Y Y Line n

Line n

Cr

Cr Cr

Cb

YCb YCb

Line n + 1

Y Y Y

Line n + 1

Cr

Cr Cr

Fig. 7.3. 4:4:4 and 4:2:2 resolution

However, the human eye cannot distinguish between horizontal and ver-
tical as far as color resolution is concerned. It is possible, therefore, to also
reduce the color resolution to one half in the vertical direction without per-
ceptible effect. MPEG-2 does this usually in one of the first steps and the
signal then becomes a 4:2:0 signal (Fig.7.4.). Four Y pixels are now in