This format preserves the progressive nature
of most 24-frame movie discs, providing a film-like, flicker-free
image with higher vertical resolution and smoother motion.
Progressive-scan video
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 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 for big-screen home-theater use by effectively doubling
the number of lines on the screen, making the scan lines that make
up the picture less visible.
Newer digital HDTVs 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. And since you get an entire image drawn at one
time--as opposed to an image split into two--a progressively scanned
video image is better than an interlaced one. This also means you
wind up with few artifacts from the interlacing process or motion
artifacts introduced 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 makes all the
difference with your movies, by helping 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 what we in the industry call 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 2:3 pull-down
(and almost all of them do) 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. (At this time, all home-theater DVD players output 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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