Cable television is a system for delivering broadcast programming to viewers by radio frequency (RF) signals transmitted through coaxial cable and through fibre-optic cable. Cable television is contrasted with terrestrial television, wherein the television signal is transmitted over-the-air by radio waves and received by a television antenna; and with satellite television, wherein a communications satellite transmits the television signal over-the-air by way of radio waves which then are received by a satellite dish antenna. A cable channel (also cable network) is television programming transmitted by cable (coaxial or optical fibre) and by satellite television. Alternative terms for a cable channel include non-broadcast channel and programming service, the latter term is used in legal contexts.
In the U.S., in 1948, in order to provide television services to the rural areas of the country where the geography blocked television signals, the Community Antenna Television (CATV) program erected large television antennas atop hills and mountains, from which the coaxial cable was directly connected to the individual houses of the community. In 1968, 6.4 percent of households in the U.S. had cable television service, 7.5 percent in 1978, 52.8 percent in 1988, and 62.4 percent of households in 1994. Moreover, FM radio programming, high-speed Internet, and telephony communication services also are transmitted by cables. In the 20th century, analog television was the television standard, but in the 21st century, cable television systems have been upgraded to digital cable operations.
Distribution To receive cable television at a given location, cable distribution lines must be available on the local utility poles or underground utility lines. Coaxial cable brings the signal to the customer's building through a service drop, an overhead or underground cable. If the subscriber's building does not have a cable service drop, the cable company will install one. The standard cable used in the U.S. is RG-6, which has a 75 ohm impedance, and connects with a type F connector. The cable company's portion of the wiring usually ends at a distribution box on the building exterior, and built-in cable wiring in the walls usually distributes the signal to jacks in different rooms to which televisions are connected. Multiple cables to different rooms are split off the incoming cable with a small device called a splitter. There are two standards for cable television: older analog cable, and newer digital cable, which can carry data signals used by digital television receivers such as high-definition television (HDTV) equipment. All cable companies in the United States have switched to or are in the course of switching to digital cable television since it was first introduced in the late 1990s.
Most cable companies require a set-top box (cable converter box) or a slot on one's TV set for conditional access module cards to view their cable channels, even on newer televisions with digital cable QAM tuners, because most digital cable channels are now encrypted, or scrambled, to reduce cable service theft. A cable from the jack in the wall is attached to the input of the box, and an output cable from the box is attached to the television, usually the RF-IN or composite input on older TVs. Since the set-top box only decodes the single channel that is being watched, each television in the house requires a separate box. Some unencrypted channels, usually traditional over-the-air broadcast networks, can be displayed without a receiver box. The cable company will provide set-top boxes based on the level of service a customer purchases, from basic set-top boxes with a standard-definition picture connected through the standard coaxial connection on the TV, to high-definition wireless digital video recorder (DVR) receivers connected via HDMI or component. Older analog television sets are cable ready and can receive the old analog cable without a set-top box. To receive digital cable channels on an analog television set, even unencrypted ones, requires a different type of box, a digital television adapter supplied by the cable company or purchased by the subscriber. Another new distribution method that takes advantage of the low-cost, high-quality DVB distribution to residential areas, uses TV gateways to convert the DVB-C, DVB-C2 stream to IP for distribution of TV over an IP network in the home. Many cable companies offer internet access through DOCSIS.
Principle of operation In the most common system of cable television, multiple television channels are distributed to subscribers through a coaxial cable, which branches from a trunkline cable that originates at the cable television headend that distributes the TV signal. Multiple TV channels can be transmitted through one coaxial cable with the technique of frequency division multiplexing. At the headend, each television channel is translated to a different frequency; by giving each channel a different frequency slot in the cable, the separate television signals do not interfere with each other. At an outdoor cable box at the residence of the TV service subscriber, the company's service drop-cable is connected to cables distributing the signal to different rooms in the house. At each television set, the subscriber's TV set or a set-top box from the cable company retranslates the desired channel to its original baseband (frequency) for display in the TV screen. Consequent to cable television piracy in the early analogue cable-TV systems, modern digital TV signals are encrypted and the set-top box must be activated with an activation code provided by the cable TV company.
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 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 businesses 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
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 electrical signal is translated into an optical signal 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 optical signal is translated back into 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.
History
The very first cable networks were operated locally, notably in 1936 by Rediffusion in London in the United KingdomRuss J. Graham, A brief history of Rediffusion and BET, Rediffusion.london. and the same year in Berlin in Germany, notably for the Olympic Games, and from 1948 onwards in the United States and Switzerland. This type of local cable network was mainly used to relay terrestrial channels in geographical areas poorly served by terrestrial television signals.
In the United States
Cable television began in the United States as a commercial business in 1950s.
The early systems simply received weak (broadcast) channels, amplified them, and sent them over unshielded wires to the subscribers, limited to a community or to adjacent communities. The receiving antenna would be taller than any individual subscriber could afford, thus bringing in stronger signals; in hilly or mountainous terrain, it would be placed at a high elevation.
At the outset, cable systems only served smaller communities without television stations of their own, and which could not easily receive signals from stations in cities because of distance or hilly terrain. In Canada, however, communities with their own signals were fertile cable markets, as viewers wanted to receive American signals. Rarely, as in the college town of Alfred, New York, U.S. cable systems retransmitted Canadian channels.
Although early (VHF) television receivers could receive 12 channels (2–13), the maximum number of channels that could be broadcast in one city was 7: channels 2, 4, either 5 or 6, 7, 9, 11 and 13, as receivers at the time were unable to receive strong (local) signals on adjacent channels without distortion. (There were frequency gaps between 4 and 5, and between 6 and 7, which allowed both to be used in the same city).
As equipment improved, all twelve channels could be utilized, except where a local VHF television station broadcast. Local broadcast channels were not usable for signals deemed to be a priority, but technology allowed low-priority signals to be placed on such channels by synchronizing their blanking intervals. TVs were unable to reconcile these blanking intervals and the slight changes due to travel through a medium, causing ghosting. The bandwidth of the amplifiers was also limited, meaning frequencies over 250 MHz were difficult to transmit to distant portions of the coaxial network, and UHF channels could not be used at all. To expand beyond 12 channels, non-standard midband channels had to be used, located between the FM band and Channel 7, or superband beyond Channel 13 up to about 300 MHz; these channels initially were only accessible using separate tuner boxes that sent the chosen channel into the TV set on Channel 2, 3 or 4. Initially, UHF broadcast stations were at a disadvantage because the standard TV sets in use at the time were unable to receive their channels. With the passage of the All-Channel Receiver Act in 1964, all new television sets were required to include a UHF tuner; nonetheless, it would still take a few years for UHF stations to become competitive.
Before being added to the cable box itself, these midband channels were used for early incarnations of pay TV, e.g. The Z Channel (Los Angeles) and HBO but transmitted in the clear i.e. not scrambled as standard TV sets of the period could not pick up the signal nor could the average consumer de-tune the normal stations to be able to receive it.
Once tuners that could receive select mid-band and super-band channels began to be incorporated into standard television sets, broadcasters were forced to either install scrambling circuitry or move these signals further out of the range of reception for early cable-ready TVs and VCRs. However, once consumer sets had the ability to receive all 181 FCC-allocated channels, premium broadcasters were left with no choice but to scramble.
The descrambling circuitry was often published in electronics hobby magazines such as Popular Science and Popular Electronics allowing anybody with anything more than a rudimentary knowledge of broadcast electronics to be able to build their own and receive the programming without cost.