563489578934

Sec. 8–9 Television 635
Composite
baseband
video input
m c (t)
Inverting
amplifier
–m c (t) AM s v (t)
transmitter
(visual) AM
visual
carrier
Vestigial
sideband
filter
s v0 (t)
VSB
Duplexer
RF out
To
antenna
Audio in
m a (t)
FM
transmitter
(aural)
s a (t)
FM aural carrier
(a) Equipment Block Diagram
T h =63.5 msec
A c [1 – 0.8 m c (t)]
s v (t)
Sync level
Blanking level
Black level
TV channel
6 MHz bandwidth
4.5 MHz
White level
1.25 MHz
4.2 MHz
3.579545 MHz
Video interval
Sync interval
f cv f sc f ca
1 MHz
Color
subcarrier
frequency
Visual
carrier
frequency
f
Aural
carrier
frequency
(b) AM Visual Carrier Waveform, sv(t)
(Typical Video Modulation Illustrated)
Figure 8–31
(c) Magnitude Spectrum of RF Output
(Frequency Response Characteristic)
TV transmission system.
This gives an overall ERP of 100 kW, which is the maximum power licensed by the FCC for
channels 2 to 6 (low-VHF band). For a UHF station operating on channel 20, some typical
values are P PEP = 21.7 kW, G A = 27, G L = 0.854 (850 ft of transmission line), which gives an
ERP of 500 kW.
A simplified block diagram of a black-and-white TV receiver is shown in Fig. 8–32.
The composite video, m c (t), as described earlier, plus a 4.5-MHz FM carrier containing the
audio modulation, appears at the output of the envelope detector. The 4.5-MHz FM carrier
is present because of the nonlinearity of the envelope detector and because the detector input
(IF signal) contains, among other terms, the FM aural carrier plus a discrete (i.e., sinusoidal)
term, which is the video carrier, located 4.5 MHz away from the FM carrier signal.
The intermodulation product of these two signals produces the 4.5-MHz FM signal, which
is called the intercarrier signal, and contains the aural modulation. Of course, if either of
these two input signal components disappears, the 4.5-MHz output signal will also disappear.
This will occur if the white level of the picture is allowed to go below, say, 10% of the
peak envelope level (sync tip level) of the AM visual signal. This is why the FCC specifies