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CATV History
Color TV
TV Introduction
TV Technology
CATV Defined

 Television Broadcast Freq. OTA
MHz Channel
54-60 2
60-66 3
66-72 4
76-82 5
82-88 6
174-180 7
180-186 8
186-192 9
192-198 10
198-204 11
204-210 12
210-216 13

TV channels broadcast

Cable Television

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 TV 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 blossum 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 (2-13). These were the original TV channels broadcast over the air and were typically allocated to the major TV networks such as CBS, NBC, ABC. For example, channel 5 was the local CBS TV station while channel 4 was NBC and channel 12 was ABC. 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 CATV used cable instead of antennas, cable television systems didn't have to worry about existing broadcasting 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."

Digital Cable
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. 

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.

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