What is Interlaced Scanning :In ordinary Picture scanning, the problem of flicker occurs. To minimize this flicker, In television system, a rate of 50 vertical lines per second is utilized. This is attained by increasing the downward scanning rate.
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So electron beam scans every alternate line instead of scanning each successive line. When the beam reaches at the end of the picture frame, then its again jumps to the top and starts scanning for the lines that were not scanned or skipped in previous scanning. So, the total lines are divided into two groups. These two groups are known as fields. The name of these two groups are:
Even Field and Odd Field. Both fields are scanned alternately. This types of scanning is known as Interlaced scanning. In this type of scanning, the flicker is reduced to a greater extent because the picture frame is covered from top to bottom at the double rate.
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In the 625 line monochrome system, we divide each frame to 312.5 lines for successful interlaced scanning. We also choose the fame repetition of 25 instead of 25 to make the field frequency equal to power line frequency of 50 Hz. .This helps us to reduce the Hum effect due to pickup from mains. In America TV system, the field frequency of 60 is used as they uses 60 Hz power supply. This brings the total number of lines scanned/sec to be the same as in the 625 line system.