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 November 1, 2007 What You Need to Know: 120Hz LCD Flat-Panel Displays
In recent years, high-definition video displays have all been 60Hz devices; that is, they display images at the rate of 60 frames per second (60fps). Television began by showing the odd-numbered lines of an image for 1/60 second, then the even-numbered lines for 1/60 second. It was the eye-brain system that knit the two frames into a single frame lasting 1/30 second. The technology is called interlaced scanning, now shortened to "i." Back in the late 1940s and early 1950s, 60fps was the fastest frame rate that was economically feasible for the mass production of picture tubes at prices people could afford. Today’s video is still based on the 60fps rate. Current hi-def displays show each 1/30-second frame for 1/60 second -- but they show each frame twice. So, even though these new video displays are 60Hz/60fps, they really show video at 30fps, just as in the old days. However, there are several reasons for processing at 60fps, depending on which technology the video display is using. The higher frame rate can minimize the motion blur that bothers some display technologies, while other technologies flash each frame twice, momentarily blackening the screen between the two flashes to improve the perceived black level. The 60Hz displays also reduce the flickering of images that bothers some people, depending on the content of the images viewed. LCD flat panels are now starting to appear that operate at 120Hz. These displays can flash new or the same image 120 times per second for even lower amounts of flicker and motion blur, and even better perceived black levels. You might assume that the 120Hz displays simply flash each frame four times, for the equivalent of 30fps, but that’s not what happens. To simultaneously improve the smoothness and sharpness of motion, video engineers have added processing to 120Hz displays that analyzes two frames from the film (24fps) or video (30fps) source, analyzes them pixel by pixel, then produces a new frame that it inserts between the two original frames. This new, manufactured frame is sharper than the frame before it or the one after, and places the image halfway between the two frames that have been analyzed. The result is that the images seem to have been captured at a frame rate twice as fast as the original source, which makes for much smoother motion onscreen. For example, say you have an otherwise stationary image in which a single object is moving -- perhaps a fast-moving football player or a race car. If you freeze a 30fps video frame, the moving object is never sharp, because the 24fps or 30fps capture rate isn’t fast enough to "freeze" the motion of the subject. This is just still photography -- you have to get images taken at a rate of 250/second (250fps) or faster to eliminate motion blur in your subject or background. The video processing in 120Hz displays interpolates new frames in 24fps and 30fps source signals and thus increases the sharpness of anything in motion; any image from any source will enjoy significant increases in perceived smoothness of motion and in the sharpness of anything moving. The sharpness of these interpolated frames improves the overall image so much that the result looks like something from a Hollywood thriller in which a license-plate number is extracted from a badly blurred photo. As part of the 120Hz hype, you might also hear that, finally, we have a solution to the "24fps movie problem." You might also hear that the fix for this problem is that 5x24=120; that is, you can show each frame of a movie five times and thus eliminate the dreaded "3:2 pulldown" required to display 24fps film sources using 30fps video technology. The dreaded 3:2 pulldown is blamed for making certain kinds of motion look jerky or juddery on video screens instead of smooth. 3:2 pulldown works by repeating 12 half-frames or six full frames (it depends on the signal format and the video processing) each second in order to keep film source’s motion looking normal and its playing time accurate when displayed by 30fps video gear. It must work pretty well; in demos of the same material at 24fps with no 3:2 pulldown and at 30/60fps with 3:2 pulldown, I saw no differences in the smoothness of motion. The press seems quick to blame judder on 3:2 pulldown in home-theater movie sources, but I’ve noted judder in movie theaters and at home from DVDs, HD DVDs, and Blu-ray discs, all in the same spots. I believe that judder is usually in the original video or film source, and is not created by the 3:2 pulldown process. So until somebody can prove otherwise, I’m not buying the story that 3:2 pulldown causes serious motion-related problems. But jerky, juddery motion does exist in many sources, and the new 120Hz displays with the new video processing I’ve described do make a big improvement, regardless of whether the source is 24fps or 30fps. Does this mean 120Hz LCD flat panels will be the display of choice for the Christmas 2007 buying season? They will be for many people, but every video display technology has improved this year. Choices will still be difficult, but 120Hz technology for LCD flat panels is an important leap forward. The new video processing and 120Hz refresh rate work so well together that they’re bound to spread to other types of displays soon. Good for us! ...Doug Blackburn
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