I can't get why there are 625 lines in a PAL scanning system. I read R.R.Gulatti's standard book. In that there is a reason like:
"Suppose there are x lines in 1 field. Then for two field its 2x.For 1 field there are 292.5 scanning lines, so for 2 fields there are 2x=292.5*2=585 & 40 other lines ( because of merging effect and all).So 585+40=625."
I searched this on net but I could not find a proper reason for this. Also in Wikipedia there is no satisfactory answer.
I can't understand why there are 625 lines or odd lines in T.V. scanning. There is a sentence that mentions that "Scanning lines are always odd" but there is no reason given.
I got this solution is it right?
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.
Best Answer
For PAL, used in Europe, part of Africa, part of South America, Asia and Australia, the number of scan lines is 625. For NTSC, used in most of the Americas and Japan, the number of scan lines is also an odd number, 525. This answer discusses the latter, as I cannot find a definitive answer why PAL uses an odd number of lines.
The National Television System Committee (NTSC, established in 1940) recommended a frame rate of 30 frames (images) per second, consisting of two interlaced fields per frame at 262.5 lines per field and 60 fields (30 frames) per second. Other standards in the final recommendation were an aspect ratio of 4:3, and frequency modulation (FM) for the sound signal (which was quite new at the time).
When the standard for color television was approved, there was a slight reduction of the frame rate from 30 frames per second to 30/1.001 (approximately 29.97) frames per second.
Each frame is composed of two fields, each consisting of 262.5 scan lines, for a total of 525 scan lines, but only 483 scan lines make up the visible raster. The remainder (the vertical blanking interval) allow for vertical synchronization and retrace. This blanking interval was originally designed to simply blank the receiver's CRT to allow for the simple analog circuits and slow vertical retrace of early TV receivers. However, some of these lines may now contain other data such as closed captioning.
The actual figure of 525 lines was chosen as a consequence of the limitations of the vacuum-tube-based technologies of the day. In early TV systems, a master voltage-controlled oscillator was run at twice the horizontal line frequency, and this frequency was divided down by the number of lines used (in this case 525) to give the field frequency (60 Hz in this case).
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.
This diagram show both the visible lines and the horizontal and vertical retrace lines. I was not aware the latter zigzagged back and forth, but have seen that on a couple of independent diagrams.
Note the half-lines, starting at the top for the odd field and ending at the bottom for the event field.
Here is a good website that describes interlaced scanning in much more detail.