Improving Picture Quality with Line Doublers and Scalers
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, 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.