THE EVOLUTION OF CATV
drops 3 dB. Then someone got the bright idea of amplifying the signal before splitting.
Now some real problems arose. One-channel amplifiers worked fine, but two channels
from two antennas with signal combining became difficult. We have begun to enter the
world of broadband amplifiers. Among the impairments we can expect from broadband
amplifiers and their connected transmission lines (coaxial cable) are the following:
Poor frequency response. Some part of the received band had notably lower levels
than other parts. This is particularly true as the frequency increases. In other words,
there was fairly severe amplitude distortion. Thus equalization became necessary.
The mixing of two or more RF signals in the system caused intermodulation products
and "beats" (harmonics), which degraded reception.
When these TV signals carried modulation, cross-modulation (Xm) products
degraded or impaired reception.
Several small companies were formed to sell these "improved" television reception
services. Some of the technicians working for these companies undertook ways of curing
the ills of broadband amplifiers.
These were coaxial cable systems, where a headend with a high tower received signals
from several local television broadcasting stations, amplified the broadband signals, and
distributed the results to CATV subscribers. A subscriber's TV set was connected to the
distribution system, and the signal received looked just the same as if it were taken off
the air with its own antenna. In fringe areas, signal quality, however, was much better
than own-antenna quality. The key to everything was that no changes were required in
the user's TV set. It was just an extension of her/his TV set antenna. This simple concept
is illustrated in Figure 17.2.
Note in Figure 17.2 that home A is in the shadow of a mountain ridge and receives
a weakened diffracted signal off the ridge and a reflected signal off the lake. Here is the
typical multipath scenario resulting in ghosts in A's TV screen. The picture is also snowy,
meaning noisy, as a result of poor carrier-to-noise ratio. Home B extended the antenna
height to be in line-of-sight of the TV transmitting antenna. Its antenna is of higher gain;
thus it is more discriminating against unwanted reflected and diffracted signals. Home B
has an excellent picture without ghosts. Home B shares its fine signal with home A by
use of a 3-dB power split (P) and a length of coaxial cable.
Early System Layouts
In Figure 17.1 we showed an early CATV distribution system (ca. 1968). Taps and cou-
plers (power splits) are not shown.
These systems provided from 5 to 12 TV channels. An
LOS microwave system might bring in channels from distant cities. We had direct expe-
rience with an Atlantic City, NJ, system where channels were brought in by microwave
from Philadelphia and New York City. A 12-channel system was derived and occupied
the entire assigned VHF band (i.e., channels 213).
As UHF TV stations began to appear, a new problem arose for the CATV operator. It
was incumbent on that operator to keep the bandwidth as narrow as possible. One approach
was to convert UHF channels to vacant VHF channel allocations at the headend.
Tap. A device for extracting a portion of the CATV signal from the cable.
Coupler. A device used to combine signals or divide signals.
Power Split or Power Splitter. A device used to divide a signal between or among paths and not necessarily