All "standard" U.S. spec television sets and some high-definition television sets use a process called "interlacing" to create the screen image. A standard U.S. spec television set uses 525 lines to create the image. A standard US-spec refreshes 60 times per second -- BUT, only half the lines are refreshed each time. The odd-numbered lines are refreshed, then 1/60th of a second later the even-numbered lines are refreshed, then 1/60th of a second later the odd-numbered lines are refreshed again, et cetera. So in reality each line is refreshed only 30 times per second. However, Computer Monitors and many high-definition TVs have what is called "progressive scan" refreshing. Instead of alternately refreshing every other line 30 times per second, progressive scan refreshes each line starting from the top and moving to the bottom. That is, it refreshes line number one, then line number two, then line number three, et cetera, all the way to the bottom. A complete top-to-bottom scan is completed 60 times per second. The primary advantages of this method are: a) movement is rendered more accurately because the entire screen refreshes 60 times per second rather than 30 times per second; b) this substantially reduces "flicker" which is more easily detected up-close, which is one of the reasons why progressive-scan is used on computer monitors. Progressive-scan is particularly common on HDTVs with 720 lines. This standard is called 720p. Another common HDTV resolution standard is 1080 lines; however, as of this writing almost all 1080-line HDTVs use interlacing. This standard is called 1080i. Progressive-scan DVD players are capable of producing a progressive-scan picture on a video viewing device (computer monitor, 720p HDTV, etc.) However, ONLY such devices are capable of making use of a DVD player's progressive scan capability. A progressive-scan DVD player viewed through an interlace-display device (such as a standard television set or a 1080i HDTV set) will still produce a picture that is refreshed using the interlace method rather than the progressive-scan method. For this reason, most people have no need for progressive-scan-capable DVD players unless they plan to purchase a 720p HDTV in the near future. That said, almost all but the cheapest "stripper" DVD players have progressive-scan capability.
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