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Technology of satellite television
Direct broadcast via satellite
Television receive-only
Early history of satellite television
Beginning of the satellite TV industry
TVRO/C-band satellite era
1990s to present of satellite television
History of Freesat
Video on demand
Reception equipment of satellite television
Technical details of satellite television



Technology of satellite television


The satellites used for broadcasting television are usually in a geostationary orbit 37,000 km (23,000 mi) above the earth's equator. The advantage of this orbit is that the satellite's orbital period equals the rotation rate of the Earth, so the satellite appears at a fixed position in the sky. Thus the satellite dish antenna which receives the signal can be aimed permanently at the location of the satellite, and does not have to track a moving satellite. A few systems instead use a highly elliptical orbit with inclination of +/-63.4 degrees and orbital period of about twelve hours, known as a Molniya orbit.

The downlink satellite signal, quite weak after traveling the great distance (see inverse-square law), is collected with 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 is a section of waveguide with a flared front-end that gathers the signals at or near the focal point and conducts them to a probe or pickup connected to a low-noise block downconverter (LNB). The LNB amplifies the signals and downconverts them to a lower block of intermediate frequencies (IF), usually in the L-band.

Designs for microstrip-based converters for amateur radio frequencies were adapted for the 4 GHz C-band. Central to these designs was concept of block downconversion of a range of frequencies to a lower, more easily handled IF.

In the United States, service providers use the intermediate frequency ranges of 9502150 MHz to carry the signal from the LNBF at the dish down to the receiver. This allows for transmission of UHF signals along the same span of coaxial wire at the same time. In some applications (DirecTV AU9-S and AT-9), ranges of the lower B-band and 22503000 MHz, are used. Newer LNBFs in use by DirecTV, called SWM (Single Wire Multiswitch), are used to implement single cable distribution and use a wider frequency range of 22150 MHz.

A practical problem relating to home satellite reception is that an LNB can basically only handle a single receiver. This is because the LNB is translating two different circular polarizations (right-hand and left-hand) and, in the case of K-band, two different frequency bands (lower and upper) to the same frequency range on the cable. Depending on which frequency and polarization a transponder is using, the satellite receiver has to switch the LNB into one of four different modes in order to receive a specific "channel". This is handled by the receiver using the DiSEqC protocol to control the LNB mode. If several satellite receivers are to be attached to a single dish, a so-called multiswitch will have to 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 are to use several dishes (or several LNBs mounted in a single dish) pointing to different satellites.

The downlinked satellite signal, weaker after traveling the great distance (see inverse-square law), is collected by using a rooftop parabolic receiving dish ("satellite dish"), which reflects the weak signal to the dish's focal point. Mounted on brackets at the dish's focal point is a feedhorn which passes the signals through a waveguide to a device called a low-noise block converter (LNB) or low noise converter (LNC) attached to the horn. 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 signal is then passed through a coaxial cable into the residence to the satellite television receiver, a set-top box next to the television.

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.

Analog television which was distributed via satellite was usually sent scrambled or unscrambled in NTSC, PAL, or SECAM television broadcast standards. The analog signal is frequency modulated and is converted from an FM signal to what is referred to as baseband. This baseband comprises the video signal and the audio subcarrier(s). The audio subcarrier is further demodulated to provide a raw audio signal.

An event called sun outage occurs when the sun lines up directly behind the satellite in the field of view of the receiving satellite dish. This happens for about a 10-minute period daily around midday, twice every year for a two-week period in the spring and fall around the equinox. During this period, the sun is within the main lobe of the dish's reception pattern, so the strong microwave noise emitted by the sun on the same frequencies used by the satellite's transponders drowns out reception.