Television is an amazing window into the world. At the flick of a button, you can travel from the North Pole to the Serengeti, watchmen 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). Radio—with pictures: The basic idea of television as "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 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 in your brain to make a moving picture (really, though they're 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 colour as well, which was a much more complex problem. Now the science of light tells us that any colour can be made by combining a mixture of the three primary colours, red, green, and blue. So the secret of making colour TV was to develop cameras that could capture separate red, green, and blue signals, transmission systems that could beam colour signals through the air, and TV sets that could turn them back into a moving, multicoloured image.
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 (in the case of a digital camera) to take a snapshot of how something appeared at a particular moment. A TV camera works differently: it has to capture a new snapshot over 24 times per second to create the illusion of a moving picture.
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 scan 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 coloured 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 colours 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 colours at once; others have three separate ones, capturing separate red, blue, and green signals—the primary colours from which any colour on your TV can be made. 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 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 fibre-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 analogue form: each signal travels as an undulating (up-and-down moving) wave. Most countries are now switching over to digital television, which works similarly to digital radio. Signals are transmitted in a numerically coded form. Many more programs can be sent this way and, generally speaking, the picture quality is better because the signals are less susceptible to interference as they travel.
TV receivers: It doesn't matter how the TV signal gets to your home: once it's arrived, your TV set treats it 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.
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The information is provided by Tecquisition for general informational and educational purposes only and is not a substitute for professional legal advice. If you have any feedback, comments, requests for technical support or other inquiries, please mail us by tecqusition@gmail.com.
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