563489578934

ƒ
ƒ
640
Wire and Wireless Communication Applications Chap. 8
frequency modulated onto a carrier, as described in Sec. 8–5. These color baseband signals
can be transmitted over communication systems that are used for analog black-and-white TV,
although the frequency response and linearity tolerances that are specified are tighter for analog
color-TV transmission.
As indicated before, the complex envelope for the subcarrier, as described by Eq. (8–66),
contains the color information. The magnitude of the complex envelope, g sc (t) is the
saturation, or amount of color, in the scene as it is scanned with time. The angle of the complex
envelope, lg sc (t), indicates the hue, or tint, as a function of time. Thus, g sc (t) is a vector that
moves about in the complex plane as a function of time as the lines of the picture are scanned.
This vector may be viewed on an instrument called a vectorscope, which has a CRT that
displays the vector g sc (t), as shown in Fig. 8–35. The vectorscope is a common piece of test
equipment that is used to calibrate analog color-TV equipment and to provide an on-line measure
of the quality of the color signal being transmitted. The usual vectorscope presentation has
x-axis
R-Y
y-axis
90°
80
100
70
0.7 0.6 0.5 0.4 0.3
110
60
120
50
130
40
Yellow
0.45 77°
Color burst phase
140
10
20
30
Red
0.64 13.7°
m i (t)
g sc (t)
150
i axis
g sc (t)
Green
0.59 151°
160
0.2
|g sc (t)|
170
0.1
33°
0° 350
0.7
180°
0.6
0.5
0.4
0.3
0.2
0.1
190
33°
340
Cyan
0.64 193.7°
Magenta
0.59 331°
200
330
q axis
210
320
m q (t)
Blue
0.45 257°
Figure 8–35 Vectorscope presentation of the complex envelope for the 3.58-MHz
chrominance subcarrier.
220
310
230
300
240
290
280
260
250
270°
B-Y