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.

What about those very inexpensive SVGA projectors? Well, if you want to enjoy HDTV, don't even consider one. Because of its much lower-resolution 800 x 600 internal panel (as compared to an XGA projector's 1024 x 768) the final picture will contain even less resolution than a standard NTSC TV signal! This is because, when an incoming 720p or 1080i HDTV wide-screen image is mapped onto the projector's panel, it needs to first be "shrunken" by 37.5% (if 720p) or 41.7% (if 1080i), so that the original 1280-pixel-wide HDTV image (if 720p) or 1920-pixel-wide image (if 1080i) will fit onto the 800-pixel-wide SVGA panel. The proportional percentage shrinkage in the vertical dimension results, in either case, in an image that no longer contains 720 or 1080 lines of resolution, but rather 450, which is less than standard TV's 480 lines!