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.
TRANSMITTING TELEVISION SIGNALS
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.
