projectisle.com.au - Image Dcan Signals

Scan Signals

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Interlaced and Progressive Scan Signals

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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).

Progressive Scan:

Interlaced Scan:	1st Scan of even lines
	2nd Scan of odd lines

Improving Picture Quality with Line Doublers and Scalars

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Unlike TV picture tubes and computer monitors, projectors don't actually "draw" the picture. Rather, at any given split-second in time they are either projecting image or not (i.e., the pixels are either "on" or "off"). Thus, an attempt to project an "interlaced" signal would result in every other line (the "odd" lines) being projected by themselves for 1/60 of a second, followed by just the even lines, resulting in a picture worse than any big screen TV.

To accommodate incoming interlaced signals such as from a TV broadcast (incl. cable), VCR, DVD or laserdisc, most projectors contain deinterlacer or "Line Doubler" circuitry that changes the interlaced signal into a progressive EDTV format. This is accomplished by waiting a full 1/30 of a second to receive both the odd and even lines before projecting them together onto the screen. During this split-second wait, the previous image frame continues to be projected a second time, so there is a fully formed image being displayed at all times.

Despite its name, there is not actually a "doubling" in the number of lines of resolution. But there is a doubling of the amount of time that each image frame is displayed, resulting in a picture that not only is devoid of "flicker," but which is also brighter.

With a high-quality line-doubler (and not all of them are), the resulting picture quality from an "interlaced" source is absolutely superb.