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Aren't both scans done 60 times per second, but progressive scan does every line and interlace only does half the lines in each scan?


There is a difference between how the picture is displayed (i.e. how it is scanned or painted onto the screen) and how much information is in the source material. For example in a signal that is 720 x 480 progressive at 30 fps (frames per second)...i.e. 480p; you get 30 full frames of information every second. The display system probably paints the same identical pixels 2 times in a row, and does a total of 60 refreshes per second. In a 480i signal you would have an input of 720 x 480 at 30 fps as well. But since it is interlaced you actually get fields of 720 x 240 at 60 fields per second. Notice that the number of new source pixels per second is the same. It is also possible for a source image to be 480p at 60 fps...but this is twice as much information per second. In a DVD, which is compressed with MPEG-2, the data is almost always 720x480 30 fps interlaced (or 480i).

Which is better? It depends on the display. In a CRT designed for interlace the persistence of the display material is chosen to show interlaced images very well. Most other display technologies as well as CRTs for PCs are optimized for progressive images. Showing an interlaced image on a progressive display (without applying some digital filter that de-interlaces the image) can show some terribly ugly artifacts.

Mike
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