On DVD players, what exactly is progressive scan and what does it do?

Answer 1

about.com has a good summary:

http://hometheater.about.com/od/beforeyoubuy/a/progressivescan.htm

Highlights:

Progressive scan pushes the limits of standard resolution and provides a more film-like image display that is more pleasing for viewing DVDs on a television display. The following is an overview of progressive scan and what it means for the home theater experience.

Interlaced Scan - The Foundation Of Traditional Video Display

Before we get into the nature of progressive scan, it is important to understand the way traditional television images are displayed on a television screen. Analog television signals, such as those from your local TV station, cable company, or VCR are displayed on a television screen using a technology known as Interlaced Scan. Basically, there are two dominant interlaced scan systems in use in the world today: NTSC and PAL.

NTSC is based on a 525-line, 60 fields/30 frames-per-second at 60Hz system for transmission and display of video images. This is an interlaced system in which each frame is scanned in two fields of 262 lines, which is then combined to display a frame of video with 525 scan lines. NTSC is the official analog video standard in the U.S., Canada, Mexico, some parts of Central and South America, Japan, Taiwan, and Korea.

PAL is the dominant format in the World for analog television broadcasting and video display (sorry U.S.) and is based on a 625 line, 50 field/25 frames a second, 50HZ system. The signal is interlaced, like NTSC, into two fields, composed of 312 lines each. Several distinguishing features are one: A better overall picture than NTSC because of the increased amount of scan lines. Two: Since color was part of the standard from the beginning, color consistency between stations and TVs are much better. In addition, PAL has a frame rate closer to that of film. PAL has a 25 frames per second rate, while film has a frame rate of 24 frames per second. Countries on the PAL system include the U.K., Germany, Spain, Portugal, Italy, China, India, most of Africa, and the Middle East.

The remainder of this article discusses the issue of progressive scan as it relates to the NTSC video system, which has special considerations with regards to the progressive scan issue.

The Development Progressive Scan

With the advent of home and office desktop computers, it was discovered that using a traditional television for the display of computer images did not yield good results, especially with text. This was due to the effect of interlaced scan. In order to produce a more pleasing and precise way of displaying images on a computer, progressive scan was developed.

Progressive scan differs from interlaced scan in that the image is displayed on a screen by scanning each line (or row of pixels) in a sequential order rather than an alternate order, as is done with interlaced scan. In other words, in progressive scan, the image lines (or pixel rows) are scanned in numerical order (1,2,3) down the screen from top to bottom, instead of in an alternate order (lines or rows 1,3,5, etc... followed by lines or rows 2,4,6). By progressively scanning the image onto a screen every 60th of a second rather than "interlacing" alternate lines every 30th of a second, a smoother, more detailed, image can be produced on the screen that is perfectly suited for viewing fine details, such as text, and is also less susceptible to interlace flicker.

Seeing this technology as way to improve the way we view images on a television screen, progressive scan has now been applied to the display of DVD and certain types of HDTV images.


CNET also did a nice piece back about 4 years ago:

http://reviews.cnet.com/4520-6449_7-5020359-1.html

Highlights:

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.

Answer 2

Its supposed to make for a smoother picture. The lines come across the screen progressively as opposed to the standard interlace system of TV broadcasts.