Asked by
disha
in
TV & Radio
at
11:25 AM on April 22, 2008
ajay bhutani's Answer
There are several numbering schemes for NTSC scan lines as seen in textbooks. A few are described below. The scheme we use is (a).
(a) Odd numbers between 1 to 483 inclusive denote lines for the first field, then odd numbers from 485 to 525 denote a vertical retrace interval, then even numbers from 2 to 482 denote the second field, then even numbers from 484 to 524 inclusive make up a vertical retrace interval.
(b) A retrace interval comes first, using odd scan lines from 1 to 41 inclusive, then odd lines 43 to 525 inclusive make up the first field, then even lines 2 to 42 make up a retrace interval, then even lines 44 to 524 make up the second field.
(c) Scan lines numbered consecutively 1 to 21 make up a retrace interval, lines numbered 22 to 263 make up the odd field, the following retrace interval consists of lines numbered 264 to 284, the lines of the even field are numbered 285 to 525.
In the above examples the picture may occupy up to 483 scan lines. If it does not occupy all 483 allowed scan lines, which extra lines are left black may vary depending on the make and model of equipment. There are other standards with slighly different numbers of active scan lines, such as:
(d) Scan lines numbered consecutively 1 to 20 make up a retrace interval, scan lines 21 to 262 and half of scan line 263 make up the odd field. The rest of line 263, lines 264 to 282, and half of line 263 make up a retrace interval, and the other half of line 263 and lines 264 to 525 make up the even field. Picture content including the half lines occupies 486 different scan lines with an "even field" (half) line uppermost and an "odd field" line lowermost.
Answered at
3:53 PM on April 23, 2008
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