432
COMMUNITY ANTENNA TELEVISION (CABLE TELEVISION)
CATV does have the capability of being a two-way system and many CATV opera-
tors are implementing such capability. The CATV band is actually the radio frequency
band topping off at about 1000 MHz (see Figure 2.6, Chapter 2, from about 1,000,000
downwards in the figure). In this case the radio signals (e.g., the TV channels) are trans-
ported in a coaxial cable medium. What a CATV operator might do is to use the band
from 50 MHz to 1 GHz for transmission outwards from the headend for TV channels
and "other services" and the band from about 5 MHz to 50 MHz to be employed from
the user back to the headend. This band will transport the other services in a full duplex
mode. These other services include POTS (plain old telephone service) and data.
In this chapter, we will describe conventional CATV, and the concept of supertrunks
including HFC (hybrid fiber-coax) systems. We will involve the reader with wideband
amplifiers in tandem, and the special impairments that we would expect to encounter
in a CATV system. System layout, hubs, and last-mile or last 100-ft considerations will
also be covered. There will also be a brief discussion of the conversion to a digital
system using some of the compression concepts covered in Chapter 16. Employing these
compression techniques, we can expect to see cable television systems delivering up to
500 TV channels to customers.
17.2
THE EVOLUTION OF CATV
17.2.1
The Beginnings
Broadcast television, as we know it, was in its infancy around 1948. Fringe area problems
were much more acute in that period. By fringe area, we mean areas with poor or scanty
signal coverage. A few TV users in fringe areas found that if they raised their antennas
high enough and improved antenna gain characteristics, an excellent picture could be
received. These users were the envy of the neighborhood. Several of these people who
were familiar with RF signal transmission employed signal splitters so that their neighbors
could share the excellent picture service. This concept is illustrated in Figure 17.2.
Soon it was found that there is a limit to how much signal splitting could be done
before signal levels got so low that they became snowy or unusable. Remember that each
time a signal splitter is added (i.e., a 50% split), neglecting insertion losses, the TV signal
Figure 17.2
CATV initial concept.
Summary :
432 COMMUNITY ANTENNA TELEVISION (CABLE TELEVISION) CATV does have the capability of being a two-way system and many CATV opera- tors are implementing such capability. Several of these people who were familiar with RF signal transmission employed signal splitters so that their neighbors could share the excellent picture service. Remember that each time a signal splitter is added (i.e., a 50% split), neglecting insertion losses, the TV signal Figure 17.2 CATV initial concept.
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cat,signal,band,system,mhz,figure,teleision,cable,other,chapter,172,fringe,serices