Concept:

• In television pictures, an effective rate of 50 vertical scans per second is utilized to reduce flicker.
• This is accomplished by increasing the downward rate of travel of the scanning electron beam so that every alternate line gets scanned instead of every successive line.
• Then, when the beam reaches the bottom of the picture frame, it quickly returns to the top to scan those lines that were missed in the previous scanning.
• Thus the total number of lines is divided into two groups called fields. Each field is scanned alternately. This method of scanning is known as interlaced scanning and is illustrated in the figure below

Diagram

• For perfect interlaced scanning it is essential that the starting points at the top of the frame is separated exactly one half line between first and second fields.
• To achieve this it is necessary to feed two regularly spaced synchronising pulses to the field time base during each frame period. For half line separation between the two fields only the topmost and the extreme bottom lines are then half lines whereas the remaining lines are all full lines.
• If there are x number of full lines per field, where x may be even or odd, the total number of full lines per frame is then 2x, an even number. To this, when the two half lines get added the total number of lines per frame becomes odd. Thus for interlaced scanning the total number of lines in any TV system must be odd.
• Therefore, in order to assist interlaced scanning, each field should have n + ½ lines, where n is a positive integer and therefore two fields or a frame should have 2n + 1 lines, which is always an odd number.