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Cable television is a system of delivering television programming to paying subscribers via radio frequency (RF) signals transmitted through coaxial cables or light pulses through fiber-optic cables. This contrasts with broadcast television, in which the television signal is transmitted over the air by radio waves and received by a television antenna attached to the television. FM radio programming, high-speed Internet, telephone service, and similar non-television services may also be provided through these cables. 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 BSkyB, as well as via IPTV, it 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, MTV, Cartoon Network, E!, Eurosport and CNN International. 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 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'' are even older as radio programming was distributed by cable in some European cities as far back as 1924. Analog television was standard in the 20th century, but since then cable systems have been upgraded to digital cable operation. == History == During the 1980s, in the United States, mandated regulations not unlike public, educational, and government access (PEG) channels created the beginning of the cable-originated live television programming. This evolved into what is known today, where many cable networks provide live cable-only broadcasts of many varieties, including cable-only produced television movies and miniseries. Live local programs with local interests were rapidly created all over the United States in most major television markets in the early 1980s. Cable television has gone through a series of evolutions in the United States and Canada. From its founding, it primarily served small communities without access to a television station, or ones with an adverse location that prevented reception of outside signals even if they were not far away, such as being in a valley. Particularly in Canada, communities with their own signals were fertile cable markets, as viewers wanted to receive American signals. Early systems carried only a maximum of seven channels, using 2, 4, 5 or 6, 7, 9, 11 and 13, as the equipment was unable to confine the signal discretely within the assigned channel bandwidth. 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 priority, but technology allowed low-priority signals to be placed on such channels by synchronizing their blanking intervals. Similarly, a local VHF station could not be carried on its broadcast channel as the signals would arrive at the TV set slightly separated in time, causing "ghosting". The bandwidth of the amplifiers also was 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. Later, the cable operators began to carry FM radio stations, and encouraged subscribers to connect their FM stereo sets to cable. Before multichannel television sound became common, Pay-TV channel sound was added to the FM cable line-ups. About this time, operators expanded beyond the 12-channel dial to use the "midband" and "superband" VHF channels adjacent to the "high band" 7-13 of North American television frequencies. Some operators as in Cornwall, Ontario, used a dual distribution network with Channels 2-13 on each of the two cables. Cable specialty channels, starting with channels oriented to show movies and large sporting or performance events, diversified into additional specialty channels, and "narrowcasting" became common. By the late 1980s, cable-only signals outnumbered broadcast signals on cable systems. Some systems were going beyond 35 channels. By the mid-1980s in Canada, cable operators were allowed by the regulator to enter into distribution contracts with cable networks. Large cable companies used addressable descramblers to limit access to premium channels to paying customers. By the 1990s, tiers became common, with customers able to subscribe to different tiers to obtain different selections of additional channels above the basic selection. During the 1990s, the pressure to accommodate the growing array of offerings resulted in digital transmission that made more efficient use of the VHF signal capacity; fibre optics was common to carry signals into areas near the home, where coax could carry higher frequencies over the short remaining distance. Although for a time in the 1980s and 1990s, television receivers and VCRs were equipped to receive the mid-band and super-band channels, such cable-ready tuners are rarely used now, requiring a return to the set-top boxes used from the 1970s onward. The conversion to digital broadcasting has put all signals - broadcast and cable - into digital form, rendering analog cable television service all but obsolete. Analog television sets are still accommodated, but their tuners are obsolete, dependent entirely on the set-top box. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cable television」の詳細全文を読む スポンサード リンク
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