Calculation of Approximate Bandwidth in 625 Line Systems:
We know that for transmission, TV picture is divided into a number of horizontal lines and each line is scanned by the electron beam of the camera tube. Each line may be further considered as consisting of a large number of picture elements, or in other words the whole picture may be thought of as consisting of a large number of picture elements.
But there is some limitation to the faithful reproduction of picture elements which arises on account of the fact that any change in light intensity of the image in vertical direction occurring in a distance less than the width of the line cannot be reproduced and thus if there is to be as much detail in the horizontal direction as in the vertical, the picture elements must be of the same size as the distance between the lines. The most detailed pattern that can be transmitted is chequer board pattern of black and white squares. With an aspect ratio (width/height) 4/3 and 625 lines every 1/25 sec., the number of elements to be scanned is given by n = 625 X (4/3 X 625) X 25.
Therefore, the time for reversal from black to white is the case is,
1/n = 3/ (625 X 4 X 625 X 25) sec.
And the time for complete cycle, black-white-black is thus.
T = (3 X 2)/ (625 X 4 X 625 X 25) sec.
And hence the frequency band covered by TV Video signal is,
Band Width (BW) = 1/T = (625 X 4 X 625 X 25)/ (3 X 2) = 6.5 MHz.
In actual practice, it is found that equal horizontal and vertical details can be obtained provided a bandwidth about 70 to 80 % of 6.5 MHz is transmitted. Therefore, in Indian TV System video bandwidth is kept is MHz.