Thread: Test pattern?
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Old 12-29-2011, 09:25 AM
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Quote:
Originally Posted by bandersen View Post
Aren't there always 525 lines (262 1/2 interlaced) vertical ? Minus a few for closed caption and other info that is.
OK, this is a very old concept, but here goes:

There are 525 total lines, but only 483 active lines due to vertical blanking. Let's just call it 480 for rough calculation (which also matches a common computer image format).
Because the lines sample the picture vertically (just like the rows of pixels in a digital picture), they cannot provide a full 480 lines of resolution for ordinary pictures - they could only do that if the picture details happened to line up exactly with the line structure. The ratio of practical viewable resolution is called the "Kell factor" after Ray Kell, who published the original research in 1934. There is an additional factor due to interlace causing interline flicker - how strong this is depends on the observer and the amount of detail contrast in the image. The first NTSC (1941) adopted an over-all factor of 0.7 for the achievable resolution compared to the number of active lines. Along with this they specified a combination of baseband video signal bandwidth (4.2 MHz) and active scan lines (486) that would give equal resolution per picture height vertically and horizontally, and also fit in a 6 MHz channel using vestigial sideband modulation and including a sound carrier. 480x0.7 = ~330, the accepted usable resolution - your mileage may vary, and there have been a lot of useless arguments over the years as to whether the useful vertical resolution number is exactly correct.

A few of the active lines were later devoted to other things like test signals, data, and closed captions, leaving 483 containing picture.

Last edited by old_tv_nut; 12-29-2011 at 09:29 AM.
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