Television converts the image from a camera lens into a stream of data that can be sent down a cable or broadcast by radio waves. It uses technology that has been developing for over a century. Many homes now get television signals from a satellite orbiting the Earth. The most important recent development, DIGITAL TELEVISION, allows people to watch a wider range of programmes and to interact with their TVs.



In the studio, a lens shines an image on to light-sensitive microchips inside a camera. The brightness of each point of the image is read from the chips to form a signal that goes to the control room. It is combined with signals from other cameras to form the complete programme. This is usually recorded, ready for broadcasting at a later date.


Giant dishes at Earth stations are used to export television programmes from the country where they were made so that people in other countries can see them. Programmes are beamed up to a satellite, which sends them to a station in the receiving country. Earth stations also send programmes to satellites that broadcast directly to homes.


Satellites like this can send TV programmes across oceans or into homes. Each satellite is like a television station on a tower 35,800 km (22,200 miles) high. Its position above the Earth never changes, making it easy to beam programmes up to it, and to receive them when the satellite sends them back to a different point on Earth. Live news is often sent by satellite.


Most people still get television signals from towers based on Earth. This is called terrestrial television. The transmitting aerial is placed high up to get its signal to as many people as possible. Terrestrial TV cannot deliver as many channels as satellite television, even when digital technology is used, because it works at lower radio frequencies.


In many cathode ray tubes, the shadow mask has vertical slots and the screen has its colours arranged in vertical stripes. Tubes like this give brighter pictures.


Cathode ray tubes are bulky, so engineers have developed two types of flat screen that can hang on a wall. Plasma screens contain thousands of tiny lamps in which electricity makes gas produce a red, green, or blue glow. Liquid crystal displays (LCDs) use thousands of tiny red, green, and blue filters in front of a white light the size of the screen.


Ordinary television transmits a new image 25 times a second, even if nothing in the picture is changing. Digital television sends out unchanging parts of the image just once. Receivers repeat these parts until they need to change them. As useless information is not transmitted, there is room for more TV channels.


Digital television set-top boxes and integrated TV sets contain computers that decode programmes. These can be used to provide other services, such as interactive TV. Viewers press remote control buttons to send commands through their phone line. They can then receive a different view of a football match, prices on a shopping channel, or the World Wide Web.

Copyright © 2007 Dorling Kindersley


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