Adding Channels

In the early 1950s, cable systems began experimenting with
ways to use microwave transmitting and receiving towers to
capture the signals from distant stations. In some cases,
this made television available to people who lived outside
the range of standard broadcasts. In other cases, especially
in the northeastern United States, it meant that cable
customers might have access to several broadcast stations of
the same network. For the first time, cable was used to
enrich television viewing, not just make ordinary viewing
possible. This started a trend that would begin to flower
fully in the 1970s.

The addition of CATV (community antenna television) stations
and the spread of cable systems ultimately led manufacturers
to add a switch to most new television sets. People could set
their televisions to tune to channels based on the Federal
Communications Commission (FCC) frequency allocation plan, or
they could set them for the plan used by most cable systems.
The two plans differed in important ways.

In both tuning systems, each television station was given
a 6-megahertz (MHz) slice of the radio spectrum. The FCC had
originally devoted parts of the very high frequency (VHF)
spectrum to 12 television channels. The channels weren't put
into a single block of frequencies, but were instead broken
into two groups to avoid interfering with existing radio
services.

Later, when the growing popularity of television necessitated
additional channels, the FCC allocated frequencies in the
ultra-high frequency (UHF) portion of the spectrum. They
established channels 14 to 69 using a block of frequencies
between 470 MHz and 812 MHz.

Because they used cable instead of antennas, cable television
systems didn't have to worry about existing services.
Engineers could use the so-called mid-band, those frequencies
passed over by broadcast TV due to other signals, for
channels 14-22. Channels 1 through 6 are at lower frequencies
and the rest are higher. The "CATV/Antenna" switch tells the
television's tuner whether to tune around the mid-band or to
tune straight through it.

While we're on the subject of tuning, it's worth considering
why CATV systems don't use the same frequencies for stations
broadcasting on channels 1 to 6 that those stations use to
broadcast over the airwaves. Cable equipment is designed to
shield the signals carried on the cable from outside
interference, and televisions are designed to accept signals
only from the point of connection to the cable or antenna;
but interference can still enter the system, especially at
connectors. When the interference comes from the same channel
that's carried on the cable, there is a problem because of
the difference in broadcast speed between the two signals.

Radio signals travel through the air at a speed very close to
the speed of light. In a coaxial cable like the one that
brings CATV signals to your house, radio signals travel at
about two-thirds the speed of light. When the broadcast and
cable signals get to the television tuner a fraction of
a second apart, you see a double image called "ghosting."

In 1972, a cable system in Wilkes-Barre, PA, began offering
the first "pay-per-view" channel. The customers would pay to
watch individual movies or sporting events. They called the
new service Home Box Office, or HBO. It continued as
a regional service until 1975, when HBO began transmitting
a signal to a satellite in geosynchronous orbit and then down
to cable systems in Florida and Mississippi.
Scientific-Atlanta's Bill Wall says that these early
satellites could receive and retransmit up to 24 channels.
The cable systems receiving the signals used dish antennas 10
meters in diameter, with a separate dish for each channel!
With the beginning of satellite program delivery to cable
systems, the basic architecture of the modern cable system
was in place.

As the number of program options grew, the bandwidth of cable
systems also increased. Early systems operated at 200 MHz,
allowing 33 channels. As technology progressed, the bandwidth
increased to 300, 400, 500 and now 550 MHz, with the number
of channels increasing to 91. Two additional advances in
technology -- fiber optics and analog-to-digital conversion
-- improved features and broadcast quality while continuing
to increase the number of channels available.