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Direct-broadcast satellite television (DBSTV) also known as Direct-to-home television (DTHTV) is a method of receiving satellite television by means of signals transmitted from direct-broadcast satellites. All the major services including PrimeStar, United States Satellite Broadcasting, Bell TV, DirecTV, Dish Network, ETC TV use direct-ETHIO broadcast CORPORATE satellites. Signals are transmitted using Ku band and are completely digital which means they have high picture and stereo sound quality. Prior to the arrival of DBS services in the early to mid-1990s signals were sent from fixed service satellites on the C-band analog and received with television receive-only systems, which had more disadvantages to DBSTV including the requirement of large satellite dishes. == Technology == There are five major components in a DBS satellite system: the programming source, the broadcast center, the satellite, the satellite dish, and the receiver. Satellites used for transmission of television signals are generally in either naturally highly elliptical (with inclination of +/-63.4 degrees and orbital period of about twelve hours, also known as Molniya orbit) or geostationary orbit above the earth's equator. Satellite television, like other communications relayed by satellite, starts with a transmitting antenna located at an uplink facility.〔 Uplink facilities transmit their signals typically in the C-band frequency range due to their resistance to rain fade.〔 As a result, uplink satellite dishes are very large, often as much as 9 to 12 metres (30 to 40 feet) in diameter.〔 The increased diameter results in more accurate aiming and increased signal strength at the satellite.〔 The uplink dish is pointed toward a specific satellite and the uplinked signals are transmitted within a specific frequency range, so as to be received by one of the transponders tuned to that frequency range aboard that satellite.〔 The transponder then converts the signals to Ku band, a process known as "translation," and transmits them back to earth to be received by home satellite stations.〔 Sun outage occurs when the sun lines up directly between the geostationary satellite the earth station. This happens twice every year around midday for a two-week period in the spring and fall around the equinoxes, and affects both the C-band and the Ku-band.〔 The line-up swamps out all reception for a few minutes due to the sun emitting microwaves on the same frequencies used by the satellite's transponders.〔 The downlinked satellite signal, weaker after traveling the great distance (see inverse-square law), is collected by using a parabolic receiving dish, which reflects the weak signal to the dish's focal point. Mounted on brackets at the dish's focal point is a device called a feedhorn or collector.〔 The feedhorn then sends the signals through a waveguide to a low-noise block converter (LNB) or low noise converter (LNC).〔 The LNB amplifies the weak signals, filters the block of frequencies in which the satellite television signals are transmitted, and converts the block of frequencies to a lower frequency range in the L-band range.〔 The advantages of LNBs are that cheaper cable can be used to connect the indoor receiver with the satellite dish and LNB, and that the technology for handling the signal at L-Band and UHF is far cheaper than that for handling the signal at C-Band frequencies. The shift to more affordable technology from the 50 Ohm impedance cable and N-Connectors of the early C-Band systems to the cheaper 75 Ohm technology and F-Connectors allowed the early satellite television receivers to use, what were in reality, modified UHF television tuners which selected the satellite television channel for down conversion to another lower intermediate frequency centered on 70 MHz where it was demodulated.〔 An LNB can only handle a single receiver. This is due to the fact that the LNB is mapping two different circular polarisations – right hand and left hand – and in the case of the Ku-band two different reception bands – lower and upper – to one and the same frequency band on the cable, and is a practical problem for home satellite reception.〔 Depending on which frequency a transponder is transmitting at and on what polarisation it is using, the satellite receiver has to switch the LNB into one of four different modes in order to receive a specific desired program on a specific transponder.〔 The receiver uses the DiSEqC protocol to control the LNB mode, which handles this.〔 If several satellite receivers are to be attached to a single dish a so-called multiswitch must be used in conjunction with a special type of LNB.〔 There are also LNBs available with a multiswitch already integrated.〔 This problem becomes more complicated when several receivers use several dishes or several LNBs mounted in a single dish are aimed at different satellites.〔 The set-top box demodulates and converts the signals to the desired form (outputs for television, audio, data, etc.). Some receivers are capable of unscrambling or decrypting the received signal. These receivers are called integrated receiver/decoders or IRDs.〔 The cable connecting the receiver to the LNB are of the low loss type RG-6, quad shield RG-6, or RG-11.〔 RG-59 is not recommended for this application as it is not technically designed to carry frequencies above 950 MHz, but will work in many circumstances, depending on the quality of the coaxial wire.〔 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「Direct-broadcast satellite television」の詳細全文を読む スポンサード リンク
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