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Defined

Progressive scan, also known as Enhanced Definition TV/EDTV/ETV, is a screen resolution for television sets defined by the Advanced Television Systems Committee, commonly used for Video Games, DVD players, and Digital Television. It provides a superior quality picture to regular analog television, by providing 480 progressive scan lines, but is not as sharp or detailed as HDTV. The screen aspect can be either 4:3 or 16:9. See also progressive scanning.

What happens in Progressive Scan?

The only difference between Progressive Scan and Standard Definition Television (SDTV) is that the scan lines are progressive instead of interlaced. Imagine this is your TV screen, divided up into scan lines:

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In ordinary interlaced TV, the television set displays one set of scan lines in the first cycle (all the 1's), and then goes back and displays the others the following cycle (the 0's). The television set alternates between these two sets while you watch. Television sets on the NTSC standard operate at 60 Hz, so each individual set is updated at about 30 Hz, or about once every 0.0333 seconds.

In progressive scan, the TV updates all the lines in one progressive sweep each cycle. This means that every single line is updated at the 60 Hz level, or about once every 0.0166 seconds.

How does it look?

Since the actual resolution is the same as ordinary television (480 scan lines), progressive scan does not offer the increases detail and clarity of HDTV. However, because the pixels are updated more frequently, the picture usually seems much sharper.

In Practice

Progressive Scan is widely implemented in digital devices such as Video Game Systems, DVD Players and Digital Television because the software stores and displays the image as a complete picture, not as alternating interlaced lines. These devices usually down sample the video to allow it to be seen on regular television sets. So they can often provide progressive scan for 'free', or without any real work. To output in higher resolutions, such as HDTV, requires that the video be rendered in these higher resolutions in the first place (which it usually isn't).

All HDTV sets support Progressive Scan as well, so do not be afraid of purchasing a product that outputs in progressive scan. It will work fine when you buy a new HDTV in the future.

A normal television screen uses interlacing to acheive a good frame rate with the limited control hardware of the 1960s. To save on costs, the steering magnets there flick the beam up and down 60 times per second -- so it could paint an entire image each time, if the horizontal steering magnets were up to it. But since a TV screen is 544 by 372 pixels, that would require them to aim the beam back and forth 337*60=20220 times. 1960s-era control circuitry couldn't do that at a reasonable price.

The solution used was to use worse phosphor for the screen, which kept glowing for longer after it was hit -- so moving images would be blurrier. In fact, the phosphor keeps glowing for a bit over a thirtieth of a second. Then, the first time the beam goes down the screen, it paints row one, then is off while the magnets reset, pulling its path back through row two, paints row three, is pulled back through row four, and so on through all the odd rows. When it reaches the bottom, it resets up to the top of the screen and starts painting the even rows. So while there's a new image every 60th of a second, you only have a totally new image every thirtieth of a second. This is called interlacing.

Computer monitors and HDTVs use progressive scan. All that means is that what I've described above doesn't happen. Instead, the entire picture is drawn at once. The benefits are:

  • Better phosphor leads to less blurring.
  • Less jitter in the beam means a cleaner image.
  • You don't have to worry about overlapping the odd rows and the even rows, so there are no interlacing artifacts.
  • Almost all progressive-scan HDTVs support digital input, so you don't torture your images through the digital-analog-digital process which happens to DVDs played on normal sets.
  • Most (but not all) progressive-scan DVD players will have 3:2 pull-down circuitry, which assists in translating the 24 fps rate of film into the 30 fps rate of video.

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