by Chris Woodford. Last updated: May 16, 2018.
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: Virtually everyone has flatscreen TVs these days, which make their pictures using LCDs, plasma, or OLEDs (organic LEDs). But until the 1990s, TVs were much bigger and bulkier and virtually all of them were using cathode-ray tube (CRT) technology, as explained below.
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).
When TV was first developed, all it could handle was black-and-white pictures; engineers struggled to figure out how to cope with color as well, which was a much more complex problem. Now the science of light tells us that any color can be made by combining a mixture of the three primary colors, red, green, and blue. So the secret of making color TV was to develop cameras that could capture separate red, green, and blue signals, transmission systems that could beam color signals through the air, and TV sets that could turn them back into a moving, multicolored image.
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 electronic light-detector (in the case of a digital camera) 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 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.
An old-fashioned 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, which is called raster scanning, turns the picture into 525 different "lines of colored light" (in a common TV system called NTSC, or 625 lines in a rival system known as PAL) that are beamed through the air to your home as a video (picture) 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 (sound) signal.
Modern TV cameras don't "scan" pictures this way anymore. Instead, just as in camcorders and webcams, their lenses focus the scene being filmed onto small, image-sensing microchips (either CCD or CMOS sensors), which convert the pattern of colors into digital, electrical signals. While traditional scanning cameras used only 525 or 625 lines, the image sensing chips in today's HDTV (high-definition television) cameras generally have either 720 or 1080 lines for capturing much more detail. Some cameras have a single image sensor capturing all colors at once; others have three separate ones, capturing separate red, blue, and green signals—the primary colors from which any color on your TV can be made.
Artwork: TV cameras break pictures up into separate red, green, and blue signals. White light (made of all colors) coming from the object being filmed passes through the lens (1) and enters a beam splitter (2). This is usually a two-part, trichroic prism that divides the light into separate red, green, and blue beams, each of which is detected by a separate CCD or CMOS image sensor. A circuit (3) mathematically synchronizes and combines the outputs from the red, green, and blue image sensors to make a single video signal based on components called luminance and chrominance (loosely, the brightness and color of each part of the image). Another part of the circuit instantly recreates the image being filmed on a small screen in a viewfinder (4). Meanwhile, sound from a microphone (not shown) is synchronized with the video signal to produce an output signal ready for transmission (5).
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. Most 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.
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.
Photos: Early TV receivers. 1) A typical black and white TV from 1949. Note the tiny screen. 2) 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
Photo: A typical old-fashioned cathode-ray-tube (CRT) television set. Virtually every TV looked like this until the 1990s, when flatscreen LCD and plasma TVs began to take over. Cathode-ray TVs are getting quite hard to find now!
Old-style, cathode-ray tube (CRT) TV sets take the incoming signal and break it into its separate audio and video 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.