There’s probably no object in your house that is a better example of the impact of science and technology than your television set — and probably no object less understood.
Strictly speaking, television really is just “radio with pictures.” Like radio, it’s based on the fact that an electrical current flowing in one wire emits electromagnetic waves that can create a current in another wire the first isn’t directly connected to.
If the original current varies in response to sounds, then the copy-cat current will vary in exactly the same way. That variance can be turned back into vibrations in the air that mimic the original sounds–and you’ve got radio. All you have to do to create television, then, is find away for those varying currents to describe a picture in addition to a sound.
But vision and hearing are quite different. We have only two eardrums, which vibrate at varying intensities, depending on the vibrations in the air around us. You could say we receive sound through no more than two “channels.” Our eyes, however, have hundreds of thousands of cells that are constantly reacting to varying wavelengths and intensities of light–which means we receive vision through hundreds of thousands of channels. To send all that information at once would require hundreds of thousands of transmitters, too.
Fortunately, someone realized you don’t have to transmit the entire scene at once. Instead, you break it down into little pieces which can be sent one after the other and reconstructed by the receiver. These little pieces are called “picture elements,” or “pixels,” and they’re created by a process called “scanning.”
Inside a black-and-white television camera is a special tube, on the front end of which is a flat glass plate coated with a material that is sensitive to light, usually a sulfur compound of antimony. Underneath the antimony coating is a positively charged metallic coating. When light strikes the antimony coating, it loses some of its electrical resistance: the brighter the light shining on it, the less resistance it has, and the more strongly it is positively charged by the metallic coating beneath it.
At the opposite end of the camera is an electron gun, which fires a narrowly focused beam of negatively charged electrons at the antimony coating, starting at the top and scanning across the plate, then moving down slightly and doing it again, and so on until it has scanned the entire plate. This negative charge neutralizes the positive charges stored on the parts of the antimony coating that have been affected by light, which changes the voltage on the metallic coating–and thus creates a fluctuating current.
That current is amplified and turned into a signal which is varied using amplitude modification (AM) (see last week’s column). That signal sets up a matching current inside the receiver, which has a tube similar to the camera tube, except the flat end is coated with a substance that glows brightly when struck by a beam of electrons. The electron gun at the back of this tube fires a narrowly focused beam of electrons that again flicks back and forth and top to bottom. The strength of the beam corresponds to the strength of the current, which corresponds to the strength of the signal, which corresponds to the amount amount of light that fell on each part of the image–and thus that image is recreated on the screen.
So when you’re watching television, all you’re really seeing a tiny point of light chasing itself across the screen. The complete picture is an illusion formed by persistance of vision, the fortunate fact that when light entering the eye is shut off, the impression of light persists for about a tenth of a second. Since the scene being televised is scanned 30 times a second, your eye sees it as a continuous whole.
Meanwhile, the sound is transmitted as an FM radio signal, and other signals are also transmitted that ensure that the television set’s picture tube is properly synchronized with the originating signal. When it’s not, the image rolls or otherwise breaks up.
Colour television adds another level of complexity: the colour camera has three tubes, filtered so that one receives only blue light, one receives only green light, and one receives only red light. The three signals created in this manner are then re-mixed into two different signals, luminance (brightness), which produces a black and white image (so that black and white sets are compatible with colour broadcasts), and chrominance, which carries the rest of the information necessary for the TV set to recreate the full colour image.
It does that by using three separate electron beams, one for red, one for green and one for blue. The screen is covered with tiny dots (or line segments) in these three colours. A mask on the back of the screen ensures that each electron beam can only cause dots of the colour it is assigned to to glow. Because all colours can be created using red, green and blue, those three colours of dots are all you need.
All this is a lot of information to transmit, of course. Television takes up a bandwidth of more than five million hertz (one hertz is one vibration per second, and the bandwidth is the difference in hertz between the highest frequency and lowest frequency used).
The first public television broadcasts began in London in 1936. In the 55 years since then, television has had an impact on our society equaled only by the automobile. It has brought us views of the planets, improved weather forecasts through the use of satellites that can send us pictures from orbit, and given us the ability to talk to face-to-face with someone halfway around the world.
It’s also brought us “Geraldo,” but hey, no technology is perfect.