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Cable Television, System Technology of - The Headend, The Wired System, Amplifiers, Network Architectures, Set-Top Boxes

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In its concept, the technology of cable television is relatively simple. It is a system of wires and amplifiers used to gather television and radio signals from a variety of sources and deliver them to the homes in a given geographic area. It is sometimes compared with the water system of a city, which takes water from one or two primary sources and distributes it to customers throughout the city. Cable television similarly distributes a roster of television channels to all the residents of an area who connect to its wire. Cable systems are expanding their services to include high-speed Internet access and traditional telephone service as well. The fundamental components of a cable system include the main office of the local system, called a “headend,” where the various signals are gathered, combined, and fed out into the system; fiber-optic lines and coaxial cables, the wires that carry the information; amplifiers that boost the signal at regular intervals and maintain signal strength; and often set-top boxes, which translate the cable signals into electronic information that the home television set can use.

The Headend

The process of getting programming to the home begins far from the headend of the local system. National and multinational corporations such as AOL-Time Warner and Disney create the programming and operate familiar channels such as CNN, ESPN, HBO, Discovery, and MTV. These companies distribute the program signals, usually by satellite, from a few main origination points, beaming the material to the more than ten thousand individual cable systems in the United States, as well as to cable systems around the globe. Large dish antennas at the headend of the local system receive these signals. The programming companies simultaneously feed their signals to other multi-channel television providers such as direct broadcast satellite (DBS) companies (e.g., Direc TV).

In addition to the basic and premium cable packages, systems carry local and regional broadcast television stations, radio stations, and national audio services. Often, they also produce their own programming or carry programs that are produced by others in the community. Local radio and television stations are picked up by powerful versions of home television antennas, or they are sometimes sent to the headend via microwave link (a specialized broadcast technology) or wire. Typically, these local broadcasters will be affiliated with and carry the major national networks (e.g., NBC, CBS, ABC, PBS, Fox, WB, and UPN). Broadcast stations that are not affiliated with national programmers, including religious stations, will also be included in the package. National audio services that feature scores of digital music channels are fed by satellite in the same manner as national video programming.

Signals from television and radio stations that are outside of the normal reception range of the system, such as stations from another part of the state, can be picked up near that station’s transmitting antenna and imported by microwave or landline. Programs that are created in television studios (usually small ones) at the headend are videotaped for later playback using professional-grade videotape machines. Those machines can also play back tapes that are created by others in the community to be carried on the public or governmental access channels of the system. Sometimes, programming will be fed by wire to the headend from a local government television facility or a television studio at an area high school or college. Many modern cable television systems also store and play back programming, usually commercials, using high-capacity digital servers.

All of this program material is electronically organized, and each signal is then imposed on a separate carrier wave, or channel. The combined signal is then sent out onto the system toward the subscriber’s home.

The Wired System

There are three types of wire that are used in modern telecommunications: the so-called twisted pair, the fiber-optic cable, and the coaxial cable. The twisted pair is the familiar wire that is used by telephone companies to carry voice and data. Compared to fiber-optic and coaxial cables, twisted pair, without special conditioning, is quite limited in the amount of information that it can carry, and it is far too narrow an electronic pipe to transmit multichannel television programming. Cable operators therefore use coaxial and fiber-optic cables.

The cable television industry derives its name from the coaxial cable. Prior to the adoption of fiber optics in the 1980s, a cable system consisted almost entirely of “coax.” The term “coaxial” refers to the two axes of the cable, a solid copper center wire (the first axis) surrounded by a metal sheath or tube (the second axis). The two axes are separated with either donut-shaped spacers or a solid, plastic-like material that is transparent to radio waves. A durable, plastic outer layer covers the cable.

Fiber is basically a thin glass thread that is about the width of a human hair. Instead of carrying information in the form of radio waves, fiber optics transmits information on beams of laser-generated light. Because it is made primarily of glass (the raw ingredients of which are plentiful) instead of copper, fiber is cheaper than coaxial cable. It can also carry significantly more information than coax and is less prone to signal loss and interference.

Both fiber and coax can carry a large number of television channels, along with other information, in part because of the way they harness the electromagnetic spectrum. The electromagnetic spectrum is the medium through which and within which television and radio signals are transmitted; it is an invisible part of the natural environment and includes such things as visible light, x-rays, gamma rays, and cosmic rays. A large portion of this natural spectrum can be employed to transmit information, and the U.S. government has allocated certain parts of it for many different types of wireless communication. This includes military communications, two-way radios, cellular telephones, and even garage-door openers. Commercial broadcasters, such as the hometown television and radio stations, therefore share this limited resource with other users.

Wired systems such as cable television, on the other hand, replicate the natural spectrum in an isolated and controlled environment. They can use all the available spectrum space that is created by that system without having to share it with other services. The amount of spectrum space that is available in a given system or for a particular application is called “bandwidth” and is measured in hertz, or more commonly, kilohertz (kHz) and megahertz (MHz). The phone line into a home is slightly more than 4 kHz, and it is termed “narrowband.” A broadcast television signal requires 6 MHz, and most modern “broadband” cable systems operate at 750 to 860 MHz, or 110-plus analog television channels.

Amplifiers

As the television signal passes through the cable lines, both fiber and coaxial, that signal loses its strength. Resistance in the coaxial cable or impurities in the fiber cause the signal to deteriorate and fade over distance. The signals, therefore, have to be amplified at regular intervals. In contemporary cable systems, these amplifiers are placed about every two thousand feet for coaxial lines; a series of amplifiers is called a “cascade.” The superior carrying power of fiber means that fewer amplifiers are needed to cover the same distance. The total number of amplifiers that can be used in a cascade or in a system is limited because every amplifier introduces a small amount of interference into the line. This interference accumulates and, with too many amplifiers, will reach a point of unacceptable distortion. The number of amplifiers that are used and the spacing between them in an actual system is depends on the system bandwidth and the medium (i.e., coaxial or fiber). A given cable system can have hundreds, even thousands, of miles of fiber and coax and hundreds of amplifiers.

The sophistication of the amplifier is also chiefly responsible for the exploitable bandwidth in the system, or the number of channels that a system can carry. The earliest cable television amplifiers could retransmit only one channel at a time, and a three-channel cable system had to have a separate set of amplifiers for each channel. Modern broadband amplifiers carry scores of channels simultaneously.

Network Architectures

The pattern in which a cable system is arranged (i.e., the configuration of wires from the headend to the subscriber’s home) is the system architecture. From the earliest days of cable in the late 1940s, the classic architecture for a cable system was known as “tree and branch.” Picture a family-tree diagram, with ancestral branches of the family coming off the trunk, and those large branches dividing and spreading out into finer and more numerous offshoots. The classic cable system is designed in this fashion. Signals leave the headend over high-capacity “trunk lines,” usually fiber optic, which wind their way through the main arteries of the community, down city streets toward local neighborhoods. “Feeder,” or distribution, cables branch off from the fiber trunk, or backbone, and spread down neighborhood streets toward hundreds, sometimes thousands, of homes. Finally, smaller coaxial “drop lines” sprout off the feeder cables to link to individual houses. All of the lines are either buried underground or strung on poles that are usually rented from the local telephone or power company. Because the trunk and feeder lines cannot support their own weight, they are lashed to heavy steel wires called “strand,” which also carry the weight of the amplifiers.

With the development of cost-effective fiber-optic technology in the 1980s, cable systems began replacing much of their coaxial line with the new, higher capacity technology, starting with the trunk lines and moving toward the feeder lines. With the change in the hardware came a change in the system architecture. Use of fiber meant reduced costs over the long term, a decrease in the number of amplifiers needed, and an increase in the overall quality of the signal. Fiber could be run directly from the headend to hubs, or nodes, serving large clusters of homes. From these fiber hubs, mini tree and branch coax systems would service area customers. This combination of fiber and coaxial cable is the hybrid fiber coax (HFC) architecture.

Set-Top Boxes

Many cable subscribers, even those who have contemporary “cable-ready” television sets, have additional cable set-top boxes, or converters, that are sitting on or next to their sets. Set-top boxes perform several important tasks for the cable system. For some television sets, especially older or non-cable-ready sets, they act as the television tuner, the device that selects the channels to be viewed. Because the wired spectrum is a closed universe, cable operators can place their channels on almost any frequency that they want, and they do so to make the most efficient use of the space and technology. Operators, for example, carry the broadcast VHF channels 2 through 13 in their “normal” place on the dial, but the UHF channels 14 through 69, which in the open spectrum are higher than and separate from the VHF channels, have been moved in “cable space.” The full cable spectrum is, in fact, divided into its own bands. Channels 2 through 6 are carried in the low band, channels 7 through 13 in the high band, and other cable network programming is distributed across the midband, superband, and hyperband channels. Part of the low band (i.e., 0 to 50 MHz) is often used to carry signals from the consumer’s home “upstream” and back to the cable company headend. Television sets that are not set up to receive the many special bands of cable require set-top boxes for the conversion.

While cable-ready television sets have taken over most of the simple functions of signal reception in modern systems, converters remain a staple in the industry for the provision of more advanced services such as premium programming and “payper-view” movies. The boxes help control the distribution of such programming to subscriber homes. Many cable systems are “addressable,” which means that each subscriber has an electronic address, and operators can turn a signal to that home on or off from the headend. The technology that helps make addressability possible is often housed in the set-top box. Finally, as cable moves into the digital era, set-top boxes are being used to convert the digital channels and services to signals that the standard analog television set can use.

Cable Interactivity and Advanced Services

While most cable systems are addressable, true interactivity remains limited in most systems. Interactivity has no set definition and can take many forms, including ordering movies when the customer wants to view them (video on demand) or having the cable system monitor the home smoke alarm. In all cases, it requires some means of getting a signal from the home back to the head-end. Cable television systems were originally configured for the efficient delivery of large amounts of programming from one point (the headend) to multiple users—a point-to-multipoint distribution scheme. The arrangement has been very successful for one-way mass distribution of content, but it is limited in its two-way capacity. As noted, cable television systems designate a small portion of their spectrum space for upstream communication, but that bandwidth has been historically underexploited by the cable industry.

In contrast, telephone systems, despite their limited bandwidth, are configured for full two-way, point-to-point communication. Unlike cable, telephone companies use a switching system to create a dedicated line between two callers. Traditional cable systems do not have the architecture or the switch to provide such service. Cable companies are seeking to overcome this technical handicap by developing techniques, using both hardware and software, to make their systems more interactive. The conversion to digital technology is especially seen as a way to provide additional and enhanced services, including interactive television, telephone service, and Internet access.

An early example of this effort is the cable modem. By distributing computer data, such as Internet web-pages, over the cable system, cable operators are able to exploit their broadband capacity and dramatically increase modem speeds. Customers who hook their computers to a cable system instead of using a standard telephone modem can download pages in seconds instead of minutes, and the cable modem is on all of the time—so there is no waiting for the computer to “dial up” an Internet connection.

Cable operators are also developing techniques that will allow them to offer telephone service using their cable plant. Ultimately, the broadband capacity of cable will provide one of the major distribution platforms for the high-speed interactive digital era—the information highway—and help create a seamless integration of video, voice, and data.

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over 6 years ago

I want know more about standard the wave pattern with f=82~1GHz, forward gain: 36dB, return gain 20dB of Trunk/Line amplifier.
You help for me, I want learning about CATV.
Thank you very much.

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