TELEVISION
The word “television” means transmitting images of a moving scene (and the sound to go with them) from the scene to a different place. A simple television system needs a television camera at the scene, a way of transmitting the images, and a television receiver where the images appear. A television camera takes 25 or 30 electronic photographs (called frames) of a scene every second and creates an electric signal that represents the colours in the frames. The receiver uses this signal to display the frames in quick succession on a screen, which creates the illusion of smooth movement because our brains merge the frames together.
The first working television system was demonstrated in 1926 by the British pioneer John Logie Baird. It created a small, wobbly image of a dummy’s face. Baird’s system used a mechanical camera and receiver. These contained large spinning discs with a spiral pattern of holes in them which divided the light from the scene into narrow lines and rebuilt it into an image. Although Baird’s system was used for experimental broadcasts, it was made redundant in the 1930s by systems that used fully electronic cameras. These were based on a device called iconoscope, developed in the USA by Vladimir Zworykin. The first broadcasting service started in Britain in 1936, with one or two hours of programmers are being shown each day.
HOW A TV SET WORKS
A television receiver (or set) builds each frame of the moving television picture line by line, using the electrical signal originally created by the camera to control the colour of the dots along the lines. In most television receivers, the picture is created by a cathode-ray tube.
The cathode-ray tube in a television receiver has a narrow neck and a flat base that forms the screen. The air is pumped out to create a vacuum. At the back of the tube are three guns that fire beams of electrons (for red, green and blue light) at the screen. Electromagnets make the beams scan quickly across the screen line by line, while the picture signal controls their strength. Where the beams hit a special coating on the inside of the screen, it gives off light. Just behind the TV screen is a plate with holes in it called a shadow mask. It ensures that the beams hit the screen only behind filters of their own colour. Filters make the light produced by the three beams appear red, blue or green. These add together to form the picture colours.
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