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A small cathode ray tube (CRT) TV set in a hotel room.

Television

by Chris Woodford. Last updated: July 11, 2013.

Television is an amazing window on the world. At the flick of a button, you can travel from the North Pole to the Serengeti, 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? Let's take a closer look!

Photo: A typical old-fashioned cathode-ray-tube (CRT) television set. These are getting quite hard to find now!.

Radio—with pictures

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.

A TV cameraman watches a recording being made by his TV camera.

Photo: A typical video/TV camera. The camera operator stands at the back watching a small TV screen that shows exactly what the camera is filming. Note that the cameraman isn't looking through the camera lens: he's seeing a recreation of what the lens is viewing on a screen (a bit like looking at the display on a digital camera). Photo by Justin R. Blake 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.

1949 black and white television Tv radiogram

Photos: Early TV receivers. Left: A typical black and white TV from 1949. Note the tiny screen. Right: An HMV 904 combined TV and radio unit from about ten years earlier. The loudspeaker is on the left, the radio tuning dial is in the center, and the TV screen (again tiny) is on the right. Both use cathode-ray tube technology and are exhibits from Think Tank, the science museum in Birmingham, England.

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 colored 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.

How a cathode-ray tube (CRT) TV works

How a television set works: A step-by-step diagram showing how three scanning electron beams draw the picture inside a cathode-ray tube CRT television

  1. An antenna (aerial) on your roof picks up radio waves from the transmitter. With satellite TV, the signals come from a satellite dish mounted on your wall or roof. With cable TV, the signal comes to you via an underground fiber-optic cable.
  2. The incoming signal feeds into the antenna socket on the back of the TV.
  3. The incoming signal is carrying picture and sound for more than one station (program). An electronic circuit inside the TV selects only the station you want to watch and splits the signal for this station into separate audio (sound) and video (picture) information, passing each to a separate circuit for further processing.
  4. The electron gun circuit splits the video part of the signal into separate red, blue, and green signals to drive the three electron guns.
  5. Someone testing and repairing a TV set
  6. The circuit fires three electron guns (one red, one blue, and one green) down a cathode-ray tube, like a fat glass bottle from which the air has been removed.
  7. The electron beams pass through a ring of electromagnets. Electrons can be steered by magnets because they have a negative electrical charge. The electromagnets steer the electron beams so they sweep back and forth across the screen, line by line.
  8. The electron beams pass through a grid of holes called a mask, which directs them so they hit exact places on the TV screen. Where the beams hit the phosphors (colored chemicals) on the screen, they make red, blue, or green dots. Elsewhere, the screen remains dark. The pattern of red, blue, and green dots builds up a colored picture very quickly.
  9. Meanwhile, audio (sound) information from the incoming signal passes to a separate audio circuit.
  10. The audio circuit drives the loudspeaker (or loudspeakers, since there are at least two in a stereo TV) so they recreate the sound exactly in time with the moving picture.

Photo: An old-style cathode-ray tube television being tested and repaired. The yellow box at the front is a meter that tests the current flowing through the TV's circuits. The opened-up TV is behind and we're looking from the back to the front (so the screen is pointing away from us). Photo by Airman Maebel Tinoko courtesy of US Navy.

Flatscreen televisions

Typical LCD TV receiver

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.

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 fuzzy 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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Text copyright © Chris Woodford 2007. All rights reserved. Full copyright notice and terms of use.

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