Electronic Communication Systems
Trade & Technology
Quiz 19 :
In communication environment, television and videos are the most important part of it. Pictures are formed in video systems with the help of scanning process. Images present in videos are separated by number of horizontal lines. On increasing the number of horizontal lines, it provides good resolution in the vertical direction. In a video system, Frame is a complete image. Number of Video Frames helps in forming a complete moving picture. That is, when screen is projecting moving pictures, then each frame is shown on screen for a very short duration of time (such as 1/25 or 1/30) and then instantly it is replaced by the next frame. In North America, the rate at which the frames are sent is 30 per second whereas in European system it is 25 per second. In a video system, Field helps in forming frame or image. In television systems, interlaced scanning is used to reduce the flicker. In order to display a picture using interlaced scan, alternate line which form the picture are transmitted first then it returns and fill the missing line. Thus, field comprises of alternate lines which form one half of the frame. So, two fields form one complete frame. Rate at which Field is transmitted is twice the rate of frame. Kindly refer the figure 19.1 from chapter 19 for interlace scanning.
In television systems, interlaced scanning is used to reduce the flicker. In order to display a picture using interlaced scan, alternate line which form the picture are transmitted first then it returns and fill in the missing line. That is, in television for interlace scan from top to bottom all the odd number of lines are drawn first then return and fill in the even number of lines. Rate at which this complete process takes place is 30 frames per second that is whole process takes place approximately 1/30 of a second. Kindly refer the figure 19.1 from chapter 19 for interlace video scanning. Following are the Advantages of Interlaced scan: 1. To transmit a television channel certain signal bandwidth is used. Now by using interlace video scanning, the signal bandwidth can be reduced by a factor of 2. 2. The systems which uses interlace scanning are easier to implement in hardware and thus helps in reducing the price of the system. Following are the Disadvantages of Interlaced scan: 1. Interlacing scanning introduces certain defects like interline flicker, line crawling and pairing. Where interline flicker is introduced, when there is enough spacing between line such that it is easily visible by human eye. Then this result in the twittering of the line structure by the alternating fields present. And also, the sharp horizontal edges of an object is not displayed properly, such that sharp edges will be present in one field but not in the other field. Whereas, because of paring defect, the visibility of line structure is increased and it reduces the image quality.
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