Page 263 - Satellite Communications, Fourth Edition

The Earth Segment 243

As previously mentioned, polarization interleaving is used, and only
half the 32 channels will be present at the input of the indoor
unit for any one setting of the antenna polarizer. This eases the job
of the tracking filter, since alternate channels are well separated in
frequency.
The selected channel is again downconverted, this time from the 950-
to 1450-MHz range to a fixed intermediate frequency, usually 70 MHz
although other values in the very high frequency (VHF) range are also
used. The 70-MHz amplifier amplifies the signal up to the levels required
for demodulation. A major difference between DBS TV and conventional
TV is that with DBS, frequency modulation is used, whereas with con-
ventional TV, amplitude modulation in the form of vestigial single side-
band (VSSB) is used. The 70-MHz, FM intermediate frequency (IF)
carrier therefore must be demodulated, and the baseband information
used to generate a VSSB signal which is fed into one of the VHF/UHF
channels of a standard TV set.
A DBS receiver provides a number of functions not shown on the sim-
plified block diagram of Fig. 8.1. The demodulated video and audio sig-
nals are usually made available at output jacks. Also, as described in
Sec. 13.3, an energy-dispersal waveform is applied to the satellite car-
rier to reduce interference, and this waveform has to be removed in the
DBS receiver. Terminals also may be provided for the insertion of IF fil-
ters to reduce interference from terrestrial TV networks, and a descram-
bler also may be necessary for the reception of some programs. The
indoor unit for digital TV is described in Chap. 16.

8.3 Master Antenna TV System
A master antenna TV (MATV) system is used to provide reception of
DBS TV/FM channels to a small group of users, for example, to the
tenants in an apartment building. It consists of a single outdoor unit
(antenna and LNA/C) feeding a number of indoor units, as shown in
Fig. 8.2. It is basically similar to the home system already described,
but with each user having access to all the channels independently
of the other users. The advantage is that only one outdoor unit is
required, but as shown, separate LNA/Cs and feeder cables are
required for each sense of polarization. Compared with the single-
user system, a larger antenna is also required (2- to 3-m diameter)
in order to maintain a good signal-to-noise ratio at all the indoor
units.
Where more than a few subscribers are involved, the distribution
system used is similar to the community antenna (CATV) system
described in the following section.