Television: Scanning and synchronising
A still picture is fundamentally an arrangement of many dark and light areas. Each small area of light or shade is called a picture element. All the elements contain the visual information in the scene. If they are transmitted and reproduced in the same degree of light or shade as original and in proper position, the picture will be reproduced.
In order to produce video signal for all the elements in the picture, it is scanned by the electron beam, one element at a time, in sequential order. The scanning is done in the same way as a written page is read to cover all the words in one line and all lines on the page. Hence, scanning is the process by which an electron beam spot is made to move across a rectangular area, so as to cover it completely. This rectangular area may be the target surface in a television camera or the screen of a picture tube in a television receiver.
The scene is scanned rapidly both in the horizontal and vertical directions simultaneously. This provides sufficient number of complete pictures or frames per second to give the illusion of continuous motion. In most of the television systems, the frame repetition rate (scanning frequency) is 25 per second.
For scanning the picture elements, saw tooth potentials can be used. Saw tooth potentials are produced by using a unijunction transistor and a R-C network. Saw tooth potentials are applied to horizontal and vertical deflector plates in a TV camera. When the saw tooth potential is applied to the horizontal plates called line synchronising pulse, the electron beam at A travels along a slanting line AB by the voltage variation of OM and reaches the point B (Fig a and b). From B, the scanning spot travels along a line BC by the voltage variation MN. In order that no picture should be scanned during the return journey (i.e. the beam from the right horizontal end to the beginning of the next line), a blanking pulse, which is a high negative potential, is applied to the control grid of electron gun during the duration of the return journey. This prevents the emission of electrons from electron gun. Then the electron beam starts to scan the next line and the process gets repeated till the whole picture is scanned. On reaching the right bottom corner, the scanning spot quickly moves up to the top left corner by the application of saw tooth potential to the vertical deflector plates, called frame synchronising pulse. Thus for scanning the picture, the three synchronising pulses are used. These synchronising pulses along with the output of the TV camera are modulated on an ultra high frequency carrier and transmitted. The accompanying sound is frequency modulated and transmitted via the same antenna.
In India, the frame repetition rate has been standardised at 25 frames per second. This repetition rate is enough to cause an illusion of continuity. But, the brightness of one frame blends (mix) smoothly into the next, through this time when the screen is blanked between successive frames. This results in definite flicker of light, that is very annoying to the observer, when the screen becomes alternatively bright and dark. To eliminate this flicker, each frame is scanned twice.
In this scanning, the total number lines are divided into two groups called fields. During the presentation of the first field, only the odd numbered lines are scanned, while during the second field all the even numbered lines are scanned. Half way along the bottom of the first field, the vertical retrace returns the scanning beam to the top of the image and completes the unfinished lines. (i.e) The remaining even numbered lines are then scanned during second field. This method of scanning is known as interlaced scanning. In the 625 line TV system, for successful interlaced scanning, the 625 lines of each frame or picture are divided into sets of 312.5 lines and each set is scanned alternatively to cover the entire picture area. The principle of interlaced scanning is shown in Fig .
Hence, with the interlaced scanning the flicker effect is eliminated without increasing the speed of scanning, which in turn does not need any increase in channel bandwidth.
Horizontal and vertical scanning frequencies
The movement of electron beam spot from left to right and back, so as to start a new line in the same direction is termed as horizontal scanning. The horizontal scanning frequency is defined as the number of lines scanned per second. In a 625 line system, transmitting 25 frames per second, the horizontal frequency is 625 × 25 = 15,625 Hz.
Consequently, time taken to scan one line is 15,625 = 1/64 µs
Vertical scanning is the movement of the electron beam spot in the vertical direction. One frame consists of two fields, resulting into 50 fields per second with a vertical field scan time of 1/50=20ms.