Television
Do you think of television as a means of wireless communication ? Let's see how television operates. You may have noticed an assortment of expensive lenses on the front of a television camera. When the camera is pointed at an object one of these lenses is forming an image of the object, as you'd expect. However, the image is not formed on film. It is formed on a screen inside an image tube.
What the image tube does is not hard to describe. How it does it is a little more difficult. The image is a pattern of light energy. The image tube changes this pattern of light energy to a corresponding pattern of electric energy.
Inside the image tube an electron gun fires a narrow beam of electrons at the image. The beam sweeps across the top of the image, from one side to the other. Then the beam drops a little and sweeps across again. This is called scanning the image. As you read these words your eyes are scanning this page, line by line, In much the same way the electron beam scans the image, line by line. The beam takes 512 lines to scan the whole image, and takes about 1/30 of a second to do the job. In one second the image is scanned completely 30 times.
As the electron beam scans the image, variations of light intensity of the image are converted into variations in electric energy. Thus the pattern of light energy becomes a corresponding pattern of electric energy. This pattern of electric energy is modulated on a carrier wave and broadcast from a transmitting station.
The antenna of a television receiver intercepts the broadcast energy pattern. The pattern is amplified, demodulated and amplified again. In the receiver is a picture tube. An electron gun in the picture tube fires a narrow beam of electrons at the picture tube screen. The screen is coated with chemicals that give off light when struck by the beam. The beam scans the picture tube screen in exactly the same manner as the beam in the image tube scans the image.
The pattern received from the transmitting station is used to vary the intensity of the electron beam that is scanning the picture tube screen. The light produced on the screen depends on the intensity of the beam. So the electron beam in the picture tube traces on the screen a pattern of light variations on the image tube screen in the camera.
In effect, the television camera takes 30 pictures of the object in front of it during one second. The television receiver presents these pictures to your eyes at the same time rate, if the object before the camera is moving, its change of position from one picture to the next appears smooth and unbroken to your eyes!
The colour television camera has three image tubes: one to handle red light from the object, another for green light, and the third for blue light. The colour television receiver has three electron guns in its picture tube, to handle the red, green, and blue energy patterns. Its picture screen is coated with dots that glow red, dots that glow blue, and dots that glow green, when struck by the beam from the right electron gun. The red-pattern gun, for instance, makes only red dots glow. If red and blue dots are made to glow together, we see purple. As you know, other colours are all made from combinations of glowing red, blue and green dots. If all three kinds of dots glow, we see white light.
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