In the 1950s, there were three television
networks in the United States. Because of the frequencies
allotted to television, the signals could only be received in
a "line of sight" from the transmitting antenna. People living
in remote areas, especially remote mountainous areas, couldn't
see the programs that were already becoming an important part
of U.S. culture.
In 1948, people living in remote valleys in Pennsylvania
solved their reception problems by putting antennas on hills
and running cables to their houses. These days, the same
technology once used by remote hamlets and select cities
allows viewers all over the country to access a wide variety
of programs and channels that meet their individual needs and
desires. By the early 1990s, cable television had reached
nearly half the homes in the United States.
Today, U.S. cable systems deliver hundreds of channels to
some 60 million homes, while also providing a growing number
of people with high-speed Internet access. Some cable systems
even let you make telephone calls and receive new programming
technologies! In this edition of HowStuffWorks,
we'll show you how cable television brings you so much
information and such a wide range of programs, from
educational to inspirational to just plain odd.
A Simple Plan
The earliest cable systems
were, in effect, strategically placed antennas with
very long cables connecting them to subscribers' television
sets. Because the signal from the antenna became weaker as
it traveled through the length of cable, cable providers had
to insert amplifiers at regular intervals to boost the
strength of the signal and make it acceptable for viewing.
According to Bill Wall, technical director for subscriber
networks at Scientific-Atlanta,
a leading maker of equipment for cable television systems,
limitations in these amplifiers were a significant issue for
cable system designers in the next three decades.
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"In a cable system, the signal might have gone through 30
or 40 amplifiers before reaching your house, one every 1,000
feet or so," Wall says. "With each amplifier, you would get
noise and distortion. Plus, if one of the amplifiers failed,
you lost the picture. Cable got a reputation for not having
the best quality picture and for not being reliable." In the
late 1970s, cable television would find a solution to the
amplifier problem. By then, they had also developed technology
that allowed them to add more programming to cable service.
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 frequencies between 88 MHz
and 174 MHz for 13 channels of programming and begin channel
14 at 216 MHz. The "CATV/Antenna" switch tells the
television's tuner whether to tune around the restricted
blocks in the FCC broadcast plan or tune "straight through"
for cable reception. In the CATV position, the switch tells
the tuner to start at 88 MHz and go straight up in 6-MHz
slices, with no break.
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."
Up in the Sky
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.
The Glass Cable
In 1976, a new sort of cable
system debuted. This system used fiber-optic cable for
the trunk cables that carry signals from the CATV
head-end to neighborhoods. The head-end is where the
cable system receives programming from various sources,
assigns the programming to channels and retransmits it onto
cables. By the late 1970s, fiber
optics had progressed considerably and so were a
cost-effective means of carrying CATV signals over long
distances. The great advantage of fiber-optic cable is that it
doesn't suffer the same signal losses as coaxial cable, which
eliminated the need for so many amplifiers. In the
early fiber-optic cable systems, the number of amplifiers
between head-end and customer was reduced from 30 or 40 down
to around six. In systems implemented since 1988, the number
of amplifiers has been further reduced, to the point that only
one or two amplifiers are required for most customers.
Decreasing the number of amplifiers made dramatic improvements
in signal quality and system reliability.
Another benefit that came from the move to fiber-optic
cable was greater customization. Since a single
fiber-optic cable might serve 500 households, it became
possible to target individual neighborhoods for messages and
services. In the 1990s, cable providers found this same
neighborhood grouping to be ideal for creating a local-area
network and providing Internet access through cable
modems.
From Analog to Digital
In 1989, General
Instruments demonstrated that it was possible to convert
an analog cable signal to digital and transmit it in a
standard 6-MHz television channel. Using MPEG
compression, CATV systems installed today can transmit up
to 10 channels of video in the 6-MHz bandwidth of a single
analog channel. When combined with a 550-MHz overall
bandwidth, this allows the possibility of nearly 1,000
channels of video on a system. In addition, digital
technology allows for error correction to ensure the quality
of the received signal. The move to digital technology also
changed the quality of one of cable television's most visible
features: the scrambled channel.
Scramble to Blue
The
first system to "scramble" a channel on a cable system was
demonstrated in 1971. In the first scrambling system,
one of the signals used to synchronize the television picture
was removed when the signal was transmitted, then reinserted
by a small device at the customer's home. Later scrambling
systems inserted a signal slightly offset from the channel's
frequency to interfere with the picture, then filtered the
interfering signal out of the mix at the customer's
television. In both cases, the scrambled channel could
generally be seen as a jagged, jumbled set of video images.
In a digital system, the signal isn't scrambled, but
encrypted. The encrypted
signal must be decoded with the proper key. Without the
key, the digital-to-analog converter can't turn the stream of
bits into anything usable by the television's tuner. When a
"non-signal" is received, the cable system substitutes an
advertisement or the familiar blue screen.
For more information on cable television and related
topics, check out the links on the next page.
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