563489578934

Sec. 8–5 Satellite Communication Systems 589
Composite video signal in
(4.5-MHz bandwidth)
To satellite
+
+
m b (t)
FM transmitter
f c = 6 GHz
(∆F) video = 10.5 MHz
(∆F) sc = 2 MHz
Audio signal in
(15-kHz bandwidth)
Figure 8–11
FM subcarrier
generator
f sc = 6.8 MHz
∆F = 75 kHz
Transmission of broadcast-quality analog TV signals from a ground station.
Dish network is testing it. DVB-S2 uses QPSK or 8PSK digital modulation with an MPEG-2
data stream. See DVB-S2 in Wikipedia for more details.
For analog TV transmission via satellite, the baseband video for a single TV channel is
frequency modulated onto a carrier. For example, to relay TV signals to the head end of CATV
systems, C-band satellites with 24 transponders are often used (as shown in Fig. 8–10). For
each transponder, the 4.5-MHz bandwidth baseband composite video signal of a single TV
channel is frequency modulated onto a 6-GHz carrier, as shown in Fig. 8–11. The composite
visual signal consists of the black-and-white video signal, the color subcarrier signals, and the
synchronizing pulse signal, as discussed in Sec. 8–9. The aural signal is also relayed over the
same transponder by frequency modulating it onto a 6.8-MHz subcarrier that is frequencydivision-multiplexed
with the composite video signal. The resulting wideband signal
frequency modulates the transmitter.
The bandwidth of the 6-GHz FM signal may be evaluated by using Carson’s rule. The
peak deviation of the composite video is 10.5 MHz, and the peak deviation of the subcarrier is
2 MHz, giving an overall peak deviation of ∆F = 12.5 MHz. The baseband bandwidth is
approximately 6.8 MHz. The transmission bandwidth is
B T = 2(¢F + B) = 2(12.5 + 6.8) = 38.6 MHz
(8–1)
which is accepted by the 36-MHz transponder. In addition, other wideband aural signals (0 to
15 kHz) can also be relayed by using FM subcarriers. Some typical subcarrier frequencies
that are used are 5.58, 5.76, 6.2, 6.3, 6.48, 6.8, 7.38, and 7.56 MHz.
Data and Telephone Signal Multiple Access
Satellite relays provide a channel for data and VF (telephone) signaling similar to conventional
terrestrial microwave radio links. That is, data may be time-division multiplexed into
DS-1, DS-2, and so on, types of signals and (digitally) modulated onto a carrier for transmission
via a satellite.