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
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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