Television
Last updated: May 21, 2007.
Television is an amazing window on the
world. At the flick of a
button, you can travel from the North Pole to the Seregenti, watch men
walking on the Moon, see athletes breaking records, or listen to world
leaders making historic speeches. Television has transformed
entertainment and education; in the United States, it's been estimated
that children spend more time watching TV (on average 1023 hours a
year) than they do sitting in school (900 hours a year). Many people
feel this is a bad thing. One of TV's inventors, Philo T
Farnsworth (1906-1971), came to the conclusion that television was
hopelessly dumbed down and refused to let his children watch it.
Whether TV is good or bad, there's no doubting that it's an ingenious
invention. But how exactly does it work?
Pictures through the air
The basic idea of television is "radio with pictures". In other
words, where radio transmits
a sound signal
(the information being broadcast) through the air, television sends a
picture signal as well. You probably know that these signals are
carried by radio waves, invisible patterns of
electricity and magnetism that race
through the air at the speed of
light (300,000 km or 186,000 miles per second). Think of the radio
waves carrying information like the waves on the sea carrying surfers:
the waves themselves aren't the information: the information surfs on
top of the waves.
Television is really a three-part invention: the TV camera that turns a picture and sound into a
signal; the TV transmitter that sends the
signal through the air; and the TV receiver (the TV set in your home)
that captures the signal and turns it back into picture and sound. TV
creates moving pictures by repeatedly capturing still pictures and
presenting these frames to your eyes so
quickly that they seem to be moving. Think of TV as an electronic
flick-book. The images are flickering on the screen so fast that they
fuse together in your brain to make a moving picture (really, though
they're really lots of still pictures displayed one after another).
TV cameras
We can see things because they reflect light into our eyes. An
ordinary "still" camera photographs
things by
capturing this light on light-sensitive film or using an electronic
light-detector to make a snapshot of how something appeared at a
particular moment. A TV
camera works in a different way: it has to capture a new snapshot over
24 times per second to create the illusion of a moving picture.
Photo: A typical video/TV camera. The operator
stands at the back watching a small TV screen that shows exactly what the camera is filming.
Public domain image courtesy of US Navy.
What's the best way for a TV camera to capture a picture? If you've
ever tried copying a masterpiece from the wall of an art
gallery into a notebook, you'll know there are lots of ways to do it.
One way is to
draw a grid of squares in your notebook, then copy the details
systematically from each area of the original picture into
the corresponding square of the grid. You could work from left to right
and from top to bottom, copying each grid square in turn. A TV camera
works exactly like this when it turns a picture into a signal for
broadcasting, only it copies the picture it sees a line at a time.
Light-detectors inside the camera scan across the picture line by line,
just like your eyes scanning from top to bottom of the picture in an
art gallery. This process turns the picture into 525 different "lines
of
colored light" that are beamed through the air to your home as a video
signal. At the same time, microphones in the TV studio capture the
sound that goes with the picture. This is transmitted alongside the
picture information as a separate audio signal.
TV transmitters
The louder you shout, the easier it is to hear someone at a
distance. Louder noises make bigger sound waves that have the power to
travel further before they get soaked up by bushes, trees, and all the
clutter around us. The same
is true of radio waves. To make radio waves that are strong enough to
carry radio and TV pictures many miles from a TV station to someone's
home, you need a really powerful transmitter. This is effectively a
giant antenna (aerial), often positioned on
top of a hill so it can
send signals as far as possible.
Not everyone receives TV signals transmitted through the air in this
way. If you have cable television, your TV pictures are "piped" into
your home down a fiber-optic cable laid
beneath your street. If you have
satellite television, the picture you see
has been bounced into space
and back to help it travel from one
side of the country to the other.
With traditional television broadcasting, picture signals are sent
in analog form: each signal travels as an
undulating (up-and-down
moving) wave. Many countries are now switching over to digital
television, which works in a similar way to digital
radio.
Signals are transmitted in a numerically coded form. Many
more programs can be sent this way and, generally speaking, picture
quality is better because the signals are less susceptible to
interference as they travel.
TV receivers
It doesn't really matter how the TV signal gets to your home: once
it's arrived, your TV set treats it exactly the same way, whether it
comes in from an antenna (aerial) on the roof, from a cable running
underground, or from a satellite dish in the garden.
Remember how a TV
camera turns the picture it's looking at into a series of lines that
form the outgoing TV signal? A TV set must work the same process in
reverse
to turn the lines in the incoming signal back into a faithful image of
the scene that the camera filmed. Different types of TV sets do this in
different ways.
Cathode-ray tube (CRT) televisions
Old-style, cathode-ray tube (CRT) TV sets take the incoming signal
and break it into its separate audio (sound) and video (picture)
components. The audio part feeds into an audio circuit, which uses a loudspeaker to recreate the original
sound
recorded in the TV studio. Meanwhile, the video signal is sent to a
separate circuit. This fires a beam of electrons
(fast-moving, negatively charged particles inside atoms) down a long
cathode-ray tube. As the beam flies down the tube, electromagnets steer
it from side to side so it scans systematically back and forth across
the screen, line by line, "painting" the picture over and over again
like a kind of invisible electronic paintbrush. The electron beam moves
so quickly that you don't see it building up the picture. It doesn't
actually "paint" anything: it makes bright spots of coloured light as
it hits different parts of the screen. That's because the screen is
coated with many tiny dots of chemicals called phosphors.
As the electron beam hits the phosphor dots, they make a tiny pinpoint
of red, blue, or green light. By switching the electron beam on and off
as it scans past the red, blue, and green dots, the video circuit can
build up an entire picture by lighting up some spots and leaving others
dark.
Flatscreen televisions
More modern LCD (liquid-crystal display)
televisions have millions of tiny picture elements called pixels that
can be switched on or off electronically to make a picture. Each pixel
is made up of three smaller red, green, and blue sub-pixels. These can
be individually turned on and off by liquid crystals—effectively
microscopic light switches that turn the sub-pixels on or off by
twisting or untwisting. Since there is no cumbersome cathode-ray tube
and phosphor screen, LCDs screens are much more compact and energy
efficient than older TV receivers.
Please see our article on flatscreen televisions
for more details.
A plasma screen is similar to an LCD, but
each pixel is effectively a microscopic fluorescent
lamp glowing with plasma. A plasma is a very hot form of gas in
which the atoms have blown apart to make negatively charged electrons
and positively charged ions (atoms minus their electrons). These move
about freely, producing a fuzy glow of light whenever they collide.
Plasma screens can be
made much bigger than ordinary cathode-ray tube televisions, but they
are also much more expensive.
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