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Progressive Scan - the Buzz Behind the Buzzword

by Jim Bray

What's the most important feature to think about when buying a DVD player?

It isn't DTS or component video outputs.

The most important consideration today is whether or not a player offers progressive scan output. Why?

Progressive scan is today's buzzword, but it's also a lot more than that. Progressively scanned output is the best way to watch a DVD short of real High Definition, yet getting progressive scan and its related topics explained to you in the store could leave your eyes glazed.

So let's make it simple.

Today's conventional TV signals offer 525 scan lines (480 of which can be seen normally), broken into two "fields" (odd and even numbered lines, which "interlaced" for form one frame). Each field is displayed 30 times per second, creating 60 fields of information per second. This 480 visible scan lines is where the terms 480i (interlaced) and 480p (progressive) come from.

The problem is that, instead of 30 true frames per second you're really getting 60 different pictures per second, each of which only contains half of the picture information. This leaves holes in the picture, which we see as a visible line structure (those black "scan lines") on the screen, or other artifacts such as flickering. This doesn't really matter when watching regular television on a 20 inch screen (unless your nose is about two inches from it), but with today's big screen TV's and high resolution video sources like DVD, the shortcomings of this NTSC system reach out and slap you in the face.

Progressive display, in a nutshell, assembles pairs of interlaced fields into one progressive frame, then shows it at least twice to ensure it takes up the same amount of time on the TV screen as two fields.

The new generations of higher end and "HD-ready" TV's, do this in different ways from a progressive scan DVD player. What they do basically boils down to "fudging" the information missing between each field's scan lines and filling it in digitally. This is generally called line doubling, though it also goes by other names such as Sony's Digital Reality Creation. And, depending on the TV, it can do an excellent job.

The result is almost no visible scan lines, less flicker, and a picture to which you can sit more closely without it becoming annoying (well, some programs are just plain annoying anyway). This can increase the "wow!" factor of a movie or sporting event, or let you count Alex Trebek's nose hairs if you're into such pastimes.

What does that have to do with DVD's? Plenty, though in truth the visible difference between progressive scan and line doubled interlaced when viewed on one of these high end TV's is subtle and may not even be noticeable unless you know what to look for. More about that later…

DVD's are designed to be shown on interlaced displays. Only programming that was originally shot on film, by a progressive TV camera (which is rare!) or which was computer-generated is inherently progressive; practically anything shot on video began life interlaced. And while DVD's are capable of storing information progressively it isn't usually done.

But since films begin life as 24 frames per second, with no interlacing, this is where progressive scan does the most good. It's also where we run into another mysterious buzzword: 3-2 pulldown.

Since TV's work at 60 fields per second and film works at 24 frames per second, the two media can never be completely compatible. So what they do when transferring film to video is add a third field to every second frame of film (hence "3-2") so the frame/field disparity evens itself out as best as possible.

That extra field stuck into every second frame is really just a repeat of the first field, and it's only there to keep the video format from throwing up its little electronic hands in confusion. In fact, that extra field isn't even stored on the DVD; instead, the player is merely instructed to play the same field twice. 3-2 pulldown is a method of restoring the original film's frame rate as best as possible on video.

3-2 pulldown works by recognizing those extra, superfluous fields and throwing them away (or, simply, not playing them) before the DVD player (or TV, if it's so equipped) combines the remaining two fields into one progressive frame. Each identical progressive frame is then repeated (three times for the frame that stretched over three fields and twice for the other), to "split the difference" between 24 frames and 60 fields per second.

Unfortunately, this still results in every second frame being 1/60th second longer than the others, which can lead to a bit of video shuddering. But that's as good as it's going to get as long as the video standard remains 60 fields per second. A standard of 72 fields per second would work better, because it divides by 24 and would lead to three progressive frames per field instead of the fraction we're now left with.

But don't hold your breath.

This is overly simplistic, of course, but it's all you need to know.

So what will you see on screen? If you're used to a conventional TV, the difference should be like night and day, with all the advantages mentioned above. If you're already using a new, high end TV that "line doubles," the difference will be more subtle and, depending on whether or not the TV has 3-2 pulldown built in, might be limited to improvements in film-originating DVD's (including more solid-appearing horizontal lines, better performance when the camera pans, and less noise on high-detail areas of the picture).

Is progressive scan worth the extra money? Well, if you have a compatible TV set or are planning to get one soon I'd say it is. Besides, progressive scan DVD players have come down in price dramatically, and can now be had for only a few hundred dollars.

Jim Bray's technology columns are distributed by the TechnoFILE and Mochila Syndicates. Copyright Jim Bray.


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