Thursday, February 26, 2009

Basics of television satellite communication/Free channels

Satellite Television has brought about revolution in the in the Television system today. Satellite Television enables one to watch Distanced stations of interest. The satellite receiver system for television are often referred to as television receive only (TVRO) because they only receives signals –not transmitting.
There are different types of Television satellite in use today. The satellite Dishes come in different sizes ; the very large dishes and the more recent smaller ones. The satellite Television communication system are used by Television cable companies to receive signals from distanced channel, they are also used by individuals directly.

In the conventional satellite Television communication, signals are obtained from a Television network or a premium channel suppliers. These signals from different source are multiplexed into just one signal which are then transmitted to a satellite in the Earth’s orbit (referred here as orbital satellite) through a process referred to as up linking. These orbital satellites orbit round the earth in 24 hr hence essentially stationary with respect to the earth. The orbital satellite contains transponders, which are special receivers that pick up the transmitted signals, amplifies it, then converts it into another frequency (usually to that of lower frequency) before transmitting it back to the television satellite antenna of a consumer or cable Television company. (Transmission back to an earth based satellite antenna is known as downlink) The satellite antenna is designed in such a way that it reflects all incoming signal to its focal point . The focal point of the satellite dish contains a horn which helps trap the majority of the signal and process it to a much lower frequency. The signal thus generated from the horn is then passed into the decoder which further process the signal and produces an output the Television set can display.

There are basically two methods of up-linking signals to an orbital satellite. The C-band and the Ku-band mode. In both cases the signal transmitted are in the radio wave region of the electromagnetic Spectrum.
C-BAND transmission
The C-band is the older mode of transmitting Television signals. The sets of desired Television programs are used to frequency-modulate a microwave carrier. Since the normal frequency of the Television programs can't travel over long distances, there is need for it to be modulated into suitable signal carrier. In this case, the carrier is a microwave with a frequency range of 5.925-6.425 GHz (the C- band region of satellite communication). These up-linked signal on getting to the orbital satellite is received by one of the transponders which amplifies and polarizes the signal horizontally or vertically and then translate the signal to a frequency range of 3.7-4.2GHz. Since the frequency with which the signal is transmitted to the satellite antenna is low, there is need to use large satellite dish for a better reception of signals.
Ku-BAND transmission / Direct broadcast satellite system (DBS)
This is the type of transmission used by the DBS system. The DBS system is a digitized communication system unlike the old conventional analog system. Here the data to be transmitted is encoded in digital format specifically in the MPEG2 format. Hence both the ground uplink stations and the Orbital satellite are equipped with the capability to handle the digital data.
Since Digital data are involved , the signal need to be transmitted in the Ku band of satellite communication.
Ku-band transmission features very high frequencies as well as a higher power satellite transponders. The digital signals containing Television programs are the used to modulate the up-linking carrier. In this case the carrier is a radio wave of super high frequency (SHF) within the range of 14.0-14.5 GHz (the Ku-band region). The signal on getting to the orbital satellite is amplified and processed to a lower frequency range of 10.95-12.75GHz. This signal is the sent to a satellite antenna on earth.
The satellites are installed in such a way that they faces a particular satellite in space. Since light travels in a straight line, signals from distance orbital satellite are reflected by the satellite’s antenna dish to its focal point which contains the antenna horn. The horn usually contains the LNBF (Low Noise Block Feed), which process the signal and converts it to the 950 -2150MHz range. This is then sent to the satellite decoder.
This higher frequencies of transmission of signal are more prone to interference by atmospheric conditions especially water vapors and fog. This interference results because water molecule absorbs maximally at this wavelength.
Generally satellite decoders process the signal from the Television satellite antenna in a way much similar to the cable decoder. The multiplexed analog and digital signals are processed to a form that can be viewed on the screen.
Many satellite Television companies acquire licenses to broadcast interesting stations from distanced and nearby television channels. They then uplinks the multiplexed signals from the source region to an orbital satellite . They usually employ the services of these commercial orbital satellites to broadcast their signals to regions where their target audience resides. Since these companies hope to make money from their services, they usually encrypt these signals so that one has to subscribe for their services before being allowed to view their programs.
In addition to these encrypted channels, the orbital satellite also transmits Television channels programs that are free to the general public. Most of these free to air channels are hosted by the television stations direct for purpose best known to them. The list of the free to air channels is growing on a daily basis. They cover almost all type of programs ranging from news, sports, entertainments, music and religious just to mention a few. Follow link to obtain a comprehensive list of the available free to air satellite and the information about the satellite that hosts them.Reference.Frenzel, L.E (2003) Principle of electronic communication system. 2nd edition. New York: McGraw hill.