Satellite Television is television delivered by the means of communications satellite and received by an outdoor antenna, usually a parabolic mirror generally referred to as a satellite dish, and as far as household usage is concerned, a satellite receiver either in the form of an external set-top box or a satellite tuner module built into a TV set. Satellite TV tuners are also available as a card or a USB stick to be attached to a personal computer. In many areas of the world satellite television provides a wide range of channels and services, often to areas that are not serviced by terrestrial or cable providers.
Direct broadcast satellite television comes to the general public in two distinct flavors – analog and digital. This necessitates either having an analog satellite receiver or a digital satellite receiver. Analog satellite television is being replaced by digital satellite television and the latter is becoming available in a better quality known as high-definition television.
Satellite TV works by broadcasting video and audio signals from geostationary satellites to satellite dishes on the Earth’s surface. These geostationary satellites orbit the earth in a region of space known as the Clarke Belt, which is approximately 22,300 miles above the equator. Each of these satellites carries a number of transponders. These transponders each carry a signal back to the Earth. These signals are typically on C Band, Ku Band, or Ka Band. The band of a signal describes, in broad terms, the frequency of the signal.
After travelling over twenty thousand miles, these signals are received by a satellite dish. This dish can be as small as 18″ across, or it can be 9′ or larger across. The purpose of the dish is to act as a collector and a reflector. The dish collects the signal and reflects it towards the feedhorn. The feedhorn receives the reflected signal and sends it to the Low Noise Blockdown converter (LNB).
The LNB amplifies the signal and converts it to a frequency more suitable for transmission over a cable. In satellite terminology, that cable is known as the IFL. The LNB transmits the signal over the IFL to the satellite receiver. The satellite receiver then sends the signal to your television set.
The advantages of using an LNB are that cheaper cable could be used to connect the indoor receiver with the satellite TV dish and LNB, and that the technology for handling the signal at L-Band and UHF was far cheaper than that for handling the signal at C-Band frequencies. The shift to cheaper 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 TV receivers to use, what were in reality, modified UHF TV tuners which selected the satellite television channel for down conversion to another lower intermediate frequency centered on 70 MHz where it was demodulated.
Digital Satellite TV:
Most satellite TV is now encoded digitally. This enables satellite broadcasters to offer more television channels using the same amount of satellite bandwidth.Satellite TV is available in both standard resolution and in the new ATSC High Definition (HDTV) format.The digital data is usually compressed with MPEG-2 or a variation thereof. MPEG-4 is beginning to replace MPEG-2 in some satellite networks.
Satellite television, like other communications relayed by satellite, starts with a transmitting antenna located at an uplink facility. Uplink satellite dishes are very large, as much as 9 to 12 meters (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 ‘retransmits’ the signals back to Earth but at a different frequency band (a process known as translation, used to avoid interference with the uplink signal), typically in the C-band (4–8 GHz) or Ku-band (12–18 GHz) or both. The leg of the signal path from the satellite to the receiving Earth station is called the downlink.
A typical satellite has up to 32 transponders for Ku-band and up to 24 for a C-band only satellite, or more for hybrid satellites. Typical transponders each have a bandwidth between 27 MHz and 50 MHz. Each geo-stationary C-band satellite needs to be spaced 2 degrees from the next satellite (to avoid interference). For Ku the spacing can be 1 degree. This means that there is an upper limit of 360/2 = 180 geostationary C-band satellites and 360/1 = 360 geostationary Ku-band satellites. C-band transmission is susceptible to terrestrial interference while Ku-band transmission is affected by rain (as water is an excellent absorber of microwaves at this particular frequency). The latter is even more adversely effected by ice crystals in thunder clouds.
The downlinked satellite signal, quite weak after traveling the great distance (see inverse-square law), is collected by 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. This feedhorn is essentially the flared front-end of a section of waveguide that gathers the signals at or near the focal point and ‘conducts’ them to a probe or pickup connected to a Low-Noise Blockdown converter or LNB. The LNB amplifies the relatively weak signals, filters the block of frequencies in which the satellite TV signals are transmitted, and converts the block of frequencies to a lower frequency range in the L-band range. The evolution of LNBs was one of necessity and invention.
Categories of usage:
· Direct broadcast via satellite
Direct broadcast satellite, (DBS) also known as “Direct-To-Home” can either refer to the communications satellites themselves that deliver DBS service or the actual television service. DBS systems are commonly referred to as “mini-dish” systems. DBS uses the upper portion of the Ku band, as well as portions of the Ka band.
· Television receive-only (TVRO)
TVRO systems are designed to receive analog and digital satellite feeds of either television or audio from both C-band and Ku-band transponders on FSS-type satellites. The higher frequency Ku-band systems tend to be Direct To Home systems and can use a smaller dish antenna because of the higher power transmissions and greater antenna gain.
· Direct to Home television
Many satellite TV customers in developed television markets get their programming through a direct broadcast satellite (DBS) provider. The provider selects programs and broadcasts them to subscribers as a set package. Basically, the provider’s goal is to bring dozens or even hundreds of channels to the customers’ television in a form that approximates the competition from Cable TV. Unlike earlier programming, the provider’s broadcast is completely digital, which means it has high picture and stereo sound quality. Early satellite television services broadcast in C-band – radio in the 3.7 GigaHertz (GHz) to 4.2 GHz frequency range. Digital broadcast satellite transmits programming in the Ku frequency range (10 GHz to 14 GHz).
Bala Aditya (MGIT – ECE 3rd year)
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