CableTv

Cable television is a system of delivering television programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fiber-optic cables. This contrasts with broadcast television (also known as terrestrial television), in which the television signal is transmitted over the air by radio waves and received by a television antenna attached to the television; or satellite television, in which the television signal is transmitted by a communications satellite orbiting the Earth and received by a satellite dish on the roof. FM radio programming, high-speed Internet, telephone services, and similar non-television services may also be provided through these cables. Analog television was standard in the 20th century, but since the 2000s, cable systems have been upgraded to digital cable operation.

A "cable channel" (sometimes known as a "cable network") is a television network available via cable television. When available through satellite television, including direct broadcast satellite providers such as DirecTV, Dish Network and Sky, as well as via IPTV providers such as Verizon FIOS and AT&T U-verse is referred to as a "satellite channel". Alternative terms include "non-broadcast channel" or "programming service", the latter being mainly used in legal contexts. Examples of cable/satellite channels/cable networks available in many countries are HBO, Cinemax, MTV, Cartoon Network, AXN, E!, FX, Discovery Channel, Canal+, Eurosport, Fox Sports, Disney Channel, Nickelodeon, CNN International, and ESPN.

The abbreviation CATV is often used for cable television. It originally stood for Community Access Television or Community Antenna Television, from cable television's origins in 1948. In areas where over-the-air TV reception was limited by distance from transmitters or mountainous terrain, large "community antennas" were constructed, and cable was run from them to individual homes. The origins of cable broadcasting for radio are even older as radio programming was distributed by cable in some European cities as far back as 1924.

Principle of operation

Diagram of a modern hybrid fiber-coaxial cable television system. At the regional headend, the TV channels are sent multiplexed on a light beam which travels through optical fiber trunklines, which fan out from distribution hubs to optical nodes in local communities. Here the light signal from the fiber is translated to a radio frequency electrical signal, which is distributed through coaxial cable to individual subscriber homes.

In the most common system, multiple television channels (as many as 500, although this varies depending on the provider's available channel capacity) are distributed to subscriber residences through a coaxial cable, which comes from a trunkline supported on utility poles originating at the cable company's local distribution facility, called the "headend". Many channels can be transmitted through one coaxial cable by a technique called frequency division multiplexing.

At the headend, each television channel is translated to a different frequency. By giving each channel a different frequency "slot" on the cable, the separate television signals do not interfere with each other. At an outdoor cable box on the subscriber's residence the company's service drop cable is connected to cables distributing the signal to different rooms in the building. At each television, the subscriber's television or a set-top box provided by the cable company translates the desired channel back to its original frequency (baseband), and it is displayed onscreen. Due to widespread cable theft in earlier analog systems, the signals are typically encrypted on modern digital cable systems, and the set-top box must be activated by an activation code sent by the cable company before it will function, which is only sent after the subscriber signs up. If the subscriber fails to pay their bill, the cable company can send a signal to deactivate the subscriber's box, preventing reception.

There are also usually "upstream" channels on the cable to send data from the customer box to the cable headend, for advanced features such as requesting pay-per-view shows or movies, cable internet access, and cable telephone service. The "downstream" channels occupy a band of frequencies from approximately 50 MHz to 1 GHz, while the "upstream" channels occupy frequencies of 5 to 42 MHz. Subscribers pay with a monthly fee. Subscribers can choose from several levels of service, with "premium" packages including more channels but costing a higher rate. At the local headend, the feed signals from the individual television channels are received by dish antennas from communication satellites. Additional local channels, such as local broadcast television stations, educational channels from local colleges, and community access channels devoted to local governments (PEG channels) are usually included on the cable service. Commercial advertisements for local business are also inserted in the programming at the headend (the individual channels, which are distributed nationally, also have their own nationally oriented commercials).

Hybrid fiber-coaxial

Main article: Hybrid fibre-coaxial

Modern cable systems are large, with a single network and headend often serving an entire metropolitan area. Most systems use hybrid fiber-coaxial (HFC) distribution; this means the trunklines that carry the signal from the headend to local neighborhoods are optical fiber to provide greater bandwidth and also extra capacity for future expansion. At the headend, the radio frequency electrical signal carrying all the channels is modulated on a light beam and sent through the fiber. The fiber trunkline goes to several distribution hubs, from which multiple fibers fan out to carry the signal to boxes called optical nodes in local communities. At the optical node, the light beam from the fiber is translated back to an electrical signal and carried by coaxial cable distribution lines on utility poles, from which cables branch out to a series of signal amplifiers and line extenders. These devices carry the signal to customers via passive RF devices called taps.