Page 283 - Satellite Communications, Fourth Edition

Analog Signals 263

Color filters are used in front of each tube to sharpen its response. In
principle, it would be possible to transmit the three color signals and at
the receiver reconstruct the color scene from them. However, this is not
the best technical approach because such signals would not be compat-
ible with monochrome television and would require extra bandwidth.
Instead, three new signals are generated which do provide compatibil-
ity and do not require extra bandwidth. These are the luminance signal
and the two chrominance signals which have been described already. The
process of generating the new signals from the color signals is mathe-
matically equivalent to having three equations in three variables and
rearranging these in terms of three new variables which are linear com-
binations of the original three. The details are shown in the matrix M
block of Fig. 9.7, and derivation of the equations from this is left as
Prob. 9.9.
At the receiver, the three color signals can be synthesized from the
luminance and chrominance components. Again, this is mathematically
equivalent to rearranging the three equations into their original form.
The three color signals then modulate the electron beams which excite
the corresponding color phosphors in the TV tube. The complete video
signal is therefore a multiplexed baseband signal which extends from
dc up to 4.2 MHz and which contains all the visual information plus syn-
chronization signals.
In conventional TV broadcasting, the aural signal is transmitted by
a separate transmitter, as shown in Fig. 9.8a. The aural information is
received by stereo microphones, split into (L R) and (L R) signals,
where L stands for left and R for right. The (L R) signal is used to
DSBSC modulate a subcarrier at 2f (31.468 kHz). This DSBSC signal
h
is then added to the (L R) signal and used to frequency modulate a
separate transmitter whose rf carrier frequency is 4.5 MHz above the
rf carrier frequency of the video transmitter. The outputs of these two
transmitters may go to separate antennas or may be combined and fed
into a single antenna, as is shown in Fig. 9.8a.
The signal format for satellite analog TV differs from that of conven-
tional TV, as shown in Fig. 9.8. To generate the uplink microwave TV
signal to a communications satellite transponder channel, the compos-
ite video signal (going from 0 Hz to about 4.2 MHz for the North
American NTSC standard) is added to two or three frequency modula-
tion (FM) carriers at frequencies of 6.2, 6.8, and/or 7.4 MHz, which carry
audio information. This composite FDM signal is then, in turn, used to
frequency modulate the uplink microwave carrier signal, producing a
signal with an rf bandwidth of about 36 MHz. The availability of three
possible audio signal carriers permits the transmission of stereo and/or
multilingual audio over the satellite link. Figure 9.8b shows a block
diagram of this system.