Cable Television

Cable television is a system of providing television to consumers via radio frequency signals transmitted to televisions through fixed optical fibers or coaxial cables . Traditionally television broadcasting (via radio waves) uses over-the-air method in which a television antenna is required. Earlier there was no technology to record programs. After the invention of camera, they recorded the program and transmitted it via an electric current to a TV set, where the antenna converted the electrical pulses back into the original images and sound. These signals were actually transmitted over radio frequencies, which travel in straight lines. So, if TV wasn’t in the “line of sight” of the source, the signal was blocked, and TV screen would be blank. As a result, the only people who had TV’s in those days (back in the 1950s), were those who lived on higher ground, or were close to the source of the program. But People who lived in the valleys of Pennsylvania (U.S) came up with a creative solution to this viewing problem. By putting up antennas on the high spots, the tallest hills, around them, and then running cables down to their homes, they could receive the signals clearly. This idea of sending signals over cables, rather than using airwaves, was the first beginnings of cable TV.

As Pennsylvania’s cable idea became recognized as a solution to the broadcast challenges of the day, cable TV technology emerged. The first challenge they had to overcome was that the original system worked well, but only within short distances. Signals sent through cables were subject to interference. Also, the longer the cable, and the farther the signal had to travel, the weaker it became and, by the time it reached the TV set, it was pretty distorted. So, to solve the distance problem, a series of amplifiers were installed along the way – about one every 1000 feet, thus increasing the strength of the signal. But the problem with this system was that every time the signal passed through an amplifier, it would pick up interference in the form of distortion and noise. And, if there was a breakdown in any of the amplifiers, there’d be no signal at all. The amplifier problems led cable TV engineers to research the transmitting of signals via microwaves. Signals were captured by receiving towers, located high enough to prevent being blocked.

The technology behind signal transmissions over the airwaves, as well as through cables is same. The total amount of all airwaves is known as the spectrum. The spectrum is separated into frequencies. Each frequency can carry a signal, which is sent in the form of an electric current. The TV signal at the source, or the origin, is converted from images and sound to an electric current, sent over the airwaves to be received by a TV antenna, which converts it back into the original images and sound. Frequencies are given values, the smallest of which is 1 hertz (Hz). In the U.S., the FCC (Federal Communications Commission) allocates certain frequencies for signals, usually in blocks. For example, a TV station is given the frequency of 6 MHz (megahertz).

UHF stands for ultra high frequency, which the FCC had designated to channels 14-69. The signals for these channels were sent between the frequencies of 470 MHz-812 MHz. TV having a CATV/Antenna switch would tell which frequency to receive your signal from and, therefore, which channel you’d see on your screen.

But with the creation of cable TV, an old challenge resurfaced – interference. And the interference was coming from the original microwave signal of the same program. The first solution to this problem was handled by splitting the VHF signals into 2 blocks, thus preventing interference from radio stations. As cable TV progressed, methods of preventing interference improved. The channels 1-6 aren’t actually carried on channels 1-6. That’s because, if they were, you could receive one signal over the airwaves, and one signal through the cable, for the same program. Since these signals travel at slightly different speeds, they’d arrive on your screen at almost the same time, but not quite. The result of this would be a double image, known as “ghosting”. So that’s why you’ll find your cable channel number to be different from the actual station number.

With the outset of satellites for communication, a whole new opportunity presented itself to the cable TV industry. Now they could import programs from broadcasters across the country, package them up, and retransmit them to your home – all via satellite. Another advantage of satellite broadcasting was the increased bandwidth. Bandwidth refers to the rate that information can be sent through a channel – the greater the bandwidth, the more information that can be sent in a given amount of time. The first cable TV systems could only transmit 33 channels because they only used frequencies up to 200 MHz. But with improved technology, bandwidths were gradually increased to 300, 400, 500 and now, even 550 MHz.

In the continued quest to cut down on interference and produce a clearer picture, improved cable technology was developed. Cable providers were importing their programming, assigning it to various channels, and re-transmitting it to their customers. The introduction of fiber-optic cable allowed them to provide this service with much-improved quality. One of the big advantages to fiber-optic cable was that it wasn’t subject to the interference challenges of the earlier coaxial cable. That meant that they didn’t need as many amplifiers on the line, either. So the bottom line was that the signal they sent to homes was much higher in quality. Fiber-optic cable also allowed cable providers to be more selective with their programming. Since just one of these cables could serve as many as 500 homes, providers could target individual communities for messages and services. This eventually led to the creation of LANs (local-area networks) which, in turn presented the opportunity for them to provide Internet access through modems. Always seeking ways to increase bandwidth capacity, cable TV technologists discovered MPEG compression, which converts signals from analog to digital. By doing this, they could transmit up to 10 channels of video in a 6 MHz bandwidth. With the overall 550-MHz bandwidth, and 10 channels broadcast within 6 MHz…well,– almost 1000 channels..