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PAL is 625 lines. 49 of those lines contain no picture information whatsoever. They contain switching information and other stuff which instructs the display on how to make the picture.
The remaining 576 lines contain picture.
Overscanning is not done so as to reduce the 625 to 576 i.e. to hide the 49 lines that contain no picture.
Overscanning is done ON the 576 lines that remain. It is an intentional and correct part of any SD TV system that 5% of the height and width of the picture (that's the 576 lines) are not displayed. A correctly adjusted display will therefore overscan by 5% in both directions and hence display something like 550 lines.
Even a "digital" display should do this. It's often the case that an SD TV picture contains junk in the extemities of the image - for example, anything sourced from or played back on tape will have headswitching distorion at the bottom. It is quite right and proper that this is hidden, even in a digital display.
I suspect, though, that the "correct" percentage of overscan will be less than 5% for High Definition, but I haven't yet seen this specified anywhere. I'm sure it is, but I haven't seen it. I doubt, though, that it will ever be zero.
Last edited by LV426; 20-07-2006 at 11:17 AM.
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