The first public demonstration of television was given by John Baird of
in 1926. the television which displays the different objects and scenes on the screen is a result of the systematic and joint efforts of the scientists. Britain
V. Zworykin of
made a significant contribution in this field. In 1928, he developed an electronic system, which superseded Baird’s mechanical system. It was a revolutionary development for telecasting programmes quickly and correctly. America
To telecast TV programmes, sound and scenes are first converted into electromagnetic waves. These waves are again converted into sound and scene by the TV set. A TV camera consists of an orthicon tube. The image of the scene formed through a lens falls on a photosensitive plate of this tube. Electrons are emitted from the plate according to the intensity of light. After that, it is canned by a cathode ray tube. Scanning changes the image into electric current. It is called a video signal. It is telecast after amplitude modulation. In addition to this, sound is converted into electric current by a microphone and is transmitted after frequency modulation. It is called an audio signal. These electro magnetic waves of scenes and sound hit our TV antenna. These are received by the TV set where they are converted back into the original scenes and sound.
The working of color TV is almost the same as a black and white TV. The light coming from the scene is divided into the three primary colors by three filters fitted in the camera. One filter allows only red color to pass the second only blue ad the third only green. The light of each color falls on different camera tubes. Each tube contains a separate glass plate and electron beam. The three signals from the tube reach the transmitter. The three signals from the tubes reach the transmitter. The TV transmitter mixes these three signals into one. A black and white signal is also mixed with this signal. After that, these signals are sent to the transmitting antenna and telecast. This signal reaches our TV sets. Three electron guns, one for red color, the second form blue and the third for green are fitted in the TV set. A layer of about 125 lakh phosphor dots of the three colors is painted on the screen of the TV set. These dots are arranged in a pattern of three and emit light when electron beam falls on them. Out of these dots, one emits red light, the second blue and the third green. The color emitted form each group of dots depends upon the intensity of electron beam. The colored scene is created on the screen by mixing of these three primary color in different ratios.
Artificial satellites brought about a revolution in the art of TV transmission. TV stations telecast programmes for different countries with the help of satellites. Nowadays, direct transmission is also done from the satellite. This is called DBS Direct Broadcast Satellites. In this system, our TV set receives signals directly from the satellite. This system requires a special dish aerial.
In cable TV, the signals reach our TV set through a cable. Nowadays, a dish aerial is installed in a colony and programmes can be viewed by the people of the colony on their TV sets through a cable. More channels can be transmitted through cable than ordinary transmission. TV signals can be sent to hills, valleys and even tall buildings with the help of the cable. Cable TV has become very popular these days.