Every television picture is made up of a number of horizontal scanning lines that "paint" an image on the screen line by line. In old-fashioned analog television, there are 525 of these lines (in North America, anyway), although not all are used for picture information; the actual image is made up of about 480 lines. Digital television can also have 480 lines (and all DVDs are 480), and the emergence of high-definition TV (HDTV) brought with it scan rates of 720 and 1080 lines, for much more detailed images.
In traditional analog (NTSC) television signals, the 480-line picture is interlaced: first all the odd lines were scanned, one after the other (1, 3, 5… 475, 477, 479) then the even ones dropped in between (2, 4, 6… 476, 478, 480). Computers use a system called progressive scanning, in which the lines are scanned in order (1, 2, 3, 4… 477, 478, 479, 480), hence the "p" in 480p and 720p. Digital TV supports both scanning methods. Familiar NTSC signals have 480 lines, interlaced, or 480i. The most common high-def formats are 720p and 1080i. Many TVs and DVD players upconvert 480i signals to a higher-resolution format, though some do a lousy job — a major reason why you may occasionally see a fancy HDTV having trouble with a regular TV signal.
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