why number of scanning lines per frame always odd in television?

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why number of scanning lines per frame always odd in television?..

Answer / jaldip jayswal

If you had an even number of lines then all lines must start
at the beginning of the frame. If you have an odd number of
lines then you will achieve the same resolution on the
screen but the half line (the odd line), split in half with
half at the start of the second frame and half at the end of
the second frame allows for an extra line period in order to
flyback vertically.

This "appears" to the human eye to be a better picture. If
you think about the lines, they are all sloping so if you
use full lines then for a full line scan the area at the top
right and bottom left have no data in for a wider area than
with half line second frame scan. The missing data area
remains the same but is now split into two parts,
immediately before the half line starts and at the end of
the first half line.
For interlaced scanning, an odd number of lines per frame
was required in order to make the vertical retrace distance
identical for the odd and even fields, which meant the
master oscillator frequency had to be divided down by an odd
number. At the time, the only practical method of frequency
division was the use of a chain of vacuum tube
multivibrators, the overall division ratio being the
mathematical product of the division ratios of the chain.
Since all the factors of an odd number also have to be odd
numbers, it follows that all the dividers in the chain also
had to divide by odd numbers, and these had to be relatively
small due to the problems of thermal drift with vacuum tube
devices. The closest practical sequence to 500 that meets
these criteria was 3 × 5 × 5 × 7 = 525.