There's a new term in video now being bantered around in home-theater circles: progressive scan. What is it? How does it work? What makes it so special? These are all good questions, especially considering that progressive-scan DVD players are now drastically dropping in price.

Interlaced Progressive Scan Interlaced video draws in a picture. Progressive scan cleans up jaggies. Reload image Reload image

To illustrate the meaning of progressive scan, let's take a look at that old analog TV in your living room. It most likely uses the interlace method to draw onscreen images. That is, the electron gun at the back of the TV tube first fires off the odd lines of the onscreen image, then during a second pass, it shoots out the even-numbered lines. This all occurs within 1/30 of a second, but what you wind up seeing is an acceptable picture that has some occasional flicker or artifacts.

To improve upon those images, sophisticated front- and rear-projection TVs have used and continue to use line doublers. Line doublers turn an interlaced NTSC picture into a progressively scanned image by effectively doubling the number of lines on the screen. As a result, the scan lines that make up the picture are less visible, and the images appear more solid.

Almost all HDTVs can draw progressive-scan pictures. Progressive scan works in the same manner as your computer monitor. It writes one full frame of video from left to right across the screen every 1/60 of a second. Since the entire image is drawn at one time--as opposed to an interlaced image where the even lines are drawn first, followed by the odd lines--a progressively scanned video image looks more stable than an interlaced one. Progressive scan also introduces fewer motion artifacts, such as jagged diagonal lines and movement in fine detail, into the picture.

Progressive-scan DVD players will work only with digital HDTVs and are not compatible with older analog sets, due to their higher horizontal-scanning frequency of 31.5kHz. One big feature that will be in any progressive-scan DVD player worth its salt is 3:2 pull-down circuitry. This tiny bit of silicon helps differentiate between the 24fps (frames per second) frame rate of film and the 30fps frame rate of video. In plain English, it smoothes out the picture and virtually eliminates jaggie artifacts.

The best example of jaggies that comes to mind is in the very beginning of the Star Trek: Insurrection DVD. The movie opens with some children playing in haystacks. Then the camera pans to a village with a number of bridges and rooftops. If you watch this scene on an HDTV with a line doubler that lacks 3:2 pull-down and a regular interlaced DVD player, you will see these nasty jaggy artifacts crawling along the bridge railings and all around the edges of the rooftops. Of course, now that you know what to look for, you'll be haunted by them in every film-based DVD you watch from now on. (Sorry.)

The other big reason why progressive-scan DVD players deliver much better pictures is because they can read extra data tags on DVDs and the players can work their image-processing magic in the digital realm before they output the video signal in analog form. (Almost all home-theater DVD players can output only an analog signal.) If you feed an interlaced DVD signal to a digital HDTV, the TV's line doubler must convert the signal to digital before processing the image, and the TV doesn't have access to the extra data stored on the DVD. For this reason, a progressive-scan DVD player can deliver a sharper, cleaner picture.

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