| Satellite TV has become the new TV carrier - particularly useful if you live in a rural area, where broadcast TV has difficulties delivering a good signal, and cable TV is nowhere to be found. So what is satellite TV, and how does it work? |
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All TV systems have to have three essential components: the broadcast center, where the TV signal is broadcast from; a carrier, that is some way of moving the TV signal from the broadcast center to the receiver; and the receiver - that is your TV.
The Broad Picture Satellite TV takes the signal from the broadcast center, and beams it up to a satellite that sits in geosynchronous orbit above the earth - geosynchronous orbit is an orbit that makes sure that the satellite will stay in one position relative to the earth.
The signal can then be bounced from the satellite to a TV dish once the TV dish is lined up with the satellite. This signal is quite diffuse, so it is "collected" by the parabolic dish, and focused onto the horn in the middle of the TV dish. The signal is then taken by cable to the receiver where it is decoded and shown on the television.
The Detail Although all satellite TV works in this way, there are some important additional technologies that are necessary in order to give a quality of service that makes satellite TV acceptable - and indeed, sometimes preferable.
Satellite TV has come into it's own with the advent of digital broadcasting. It has some disadvantages - there can be as much as a one-second delay in transmission because of the distances covered, and some coding issues, but for most TV purposes, this is acceptable.
- Compression. When the broadcast center receives TV programs, they are in a high quality video format. However, this format needs a lot of information to describe the picture, motion, color, luminescence and sound that make up your TV format.
New digital techniques have allowed the amount of information needed to convey all these facts to be decreased significantly, and these techniques conform to a generally agreed and accepted set of standards called MPEG. At the moment, most satellite TV uses MPEG2.
MPEG compression allows the moving picture to be described in groups of 11 frames, coding the first and the last frame, and then simply describing the differences between the intervening frames. This reduces the amount of information by a factor of as much as 100, and when the signal is decoded by the receiver, the image is good.
Of course, if there is a lot of motion in the program, more information is transmitted than if it is a news broadcast, where the picture is mainly of one person sitting still. But compression allows the signal to be significantly "narrower" than a full analog signal.
- Encryption. As most satellite TV is pay TV, it is necessary to make sure that only the people who have paid for the service can use it. There are therefore certain encryption techniques used to keep it secure, so that only a receiver with the correct decryption algorithm can decode the signal. Much like cable, in which someone can use a simple cable descrambler, satellite TV can still be illegally unscrambled.
As interactive TV becomes more popular, satellite TV has the ability and the technology to allow the user to take advantage of new developments. This is a TV system that delivers when other systems cannot.