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Interlaced and Progressive Scan Signals 
Standard 480-line NTSC TV broadcasts (incl. cable television), VCR, DVD and laserdisc signals are sent in an "Interlaced Scan" format. A TV screen first draws the image's odd lines, one at a time sequentially from top to bottom (which takes 1/60 of a second), and then fills in the even lines (taking another 1/60 of a second). That is, the full picture (top to bottom) is first drawn with half its information hollowed out, and then the other half is filled in -- the entire process taking 1/30 of a second.
A newer and superior scanning method called "Progressive" permits the entire picture to be drawn sequentially from top to bottom without the odd/even interlacing. Some newer DVD players now have outputs for both an interlaced and progressive scan image. And HDTV signals are now being broadcast in both progressive and interlaced formats: 720p (720 lines of resolution in progressive scan format) and 1080i (interlaced).