Television (Part 1)
Now-a-Days television is very common devise which is available in every place like homes, shops, Theaters etc. basically TELEVISION composition of two words, tele and vision. Tele means Fare (Door) and Vision mean Image (Tasveer), means the machine which is used to see the objects which are very fare. In radio we hear only sound but in Television we can See picture also.
Television depends like radio on the production of suitable electromagnetic signal that be transmitted, received and converted back into its original pattern. We know that in sound transmission the chain of events begins with a microphone, which changes sound-wave into electrical signals and ends with a loudspeaker, which converts the electrical pattern back into sound waves and we can hear the original sounds. In picture transmission the key unit is the CAMERA which looks at a scene and changes the image into electrical impulses. At the other end the equivalent of the loudspeaker is the cathode-ray tube, which converts the electrical impulses into a visible image, and then we are able to see the picture on television.
Transmission of Television
The camera is a light sensitive plate. An image of outside scene is formed on this plate by means of a lens. The material of the plate is such that electrons are released wherever light falls on it, this process is called photo electric effect. Brightly illuminated areas produce most electrons and darker areas produce proportionately less. The electrons travel to another plate called the target where they produce a pattern of positive electrical charges equivalent to the original pattern of light.
A Gun is used for the bombardment of electrons on the screen of television. Gun form steady electron beam scans the target moving rhythmically across from left of right in narrow, horizontal bands. At the end of each and it flicks rapidly back to the left side and shifts down a few millimeters to start the next band. When it reaches the bottom right-hand corner it flicks back up to near the top left and scans a second set of bands between those of the first sequences. Keep in mind that the whole scanning process takes only 1/25th of a second means 25th portion of a second and is repeated continuously.
Electrons in this process play the important role and scanning beam fall on an area of the target containing a large positive charge, they are attracted and held. In other areas a large number of electrons are reflected back to a collector. As at the end current now into the collector is proportional to the charge on the target and so to the brightness of the original image is found.
In the television current also play the important role which is amplified and fed to the transmitter where synchronizing pulses are added that will lock the scanning beam of the receiver in step with that of the camera the sound signal is added before the final output is fed to the transmitting aerial. For transmission very high VHF and Ultra High UHF radio frequencies are used.
TO BE CONTINUED……
In the next lecture we will discuss the Reception and the color of television.
