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THE FRONT LINE: DECEMBER 20, 2004 |
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JUST HOW IMPORTANT IS 1920x1080 IMAGING IN A DISPLAY? If you follow the electronic display market as I do, you know that one of the hot topics now is ‘1080p’. More specifically, it means having a front projector, rear-projection monitor, or flat-panel display with true 1920x1080 pixel progressive-scan imaging. 1920x1080 is significant for a number of reasons. The first is that it is a legacy HDTV format that evolved from the original 1152-line analog standard used in Japan. It’s also a ‘common’ display standard for both televisions and personal computers. Finally, 1920x1080 interlaced television is the most popular picture format for distribution of HDTV programs via satellite, cable, or over-the-air. Hmmm. It would seem that everyone wants (or should want) to have electronic displays that are capable of showing every one of those 2,073,600 pixels, and that anything with lower resolution would be a compromise. But would it? Right now, the most common display formats for computer screens and TVs are all grouped around 720 to 768 vertical lines/pixels. LCD TVs all have 1280x768 (Wide XGA) resolution. Plasma monitors from 42 inches to 65 inches have 1024x768 non-square or 1280/1365/1366x768 pixel matrices. Rear-projection HDTVs come with microdisplays that use 1280x768 or 1280x720 imaging devices. Widescreen front projectors for home theater and commercial use are equipped with 1280x720 and 1366x768 chips and panels. There’s plenty of product out there that can support native 720p (921,600 total pixels) HD programs, and of course all of these products will show 1080i as well with some pixel decimation (about 29%) to fit the smaller screen. So why aren’t these products sufficient? To best answer the question of how much resolution is required to show HDTV, bear in mind that our current analog TV system assumed way back in the early 1940s that TV screen sizes would never exceed 20 inches diagonally (that works out to about 12 inches vertically with a 4:3 aspect ratio). The optimal seating distance was then calculated to be 7.1 times the screen height when showing 525-line interlaced video, or around 8 feet with a 20-inch TV. It was determined that at that distance, the human eye would not be able to make out the interlaced picture scan lines and the resulting images would appear to be smooth with ‘high resolution’. Consider that at a viewing distance of 12 inches, the normal visual acuity of the human eye is 0.0035 inch. At 120 inches (10x), that number would be predicted to drop by a factor of ten (.035). In a 50-inch plasma display with an array of 1366x768 pixels, the pitch (size) of individual pixels is typically less than 1 millimeter (about .9mm), which equals .039 inches. Do the math, and you’ll see that standing 10 feet from a 50-inch plasma means you can barely perceive the HD pixel structure, and that’s only if you have 20-20 or better vision. To jam 1920x1080 pixels into that same 50-inch screen size means we’d have to shrink the pitch of each pixel to about .0276 inches. And I’ll bet you the average person could not tell the difference when all was said and done. Remember that the optimal viewing distance from an HD (1920x1080) display is 3.1x the screen height. In other words, you can sit farther back, but you should not sit any closer in order not to see picture scan lines. At a distance of 10 feet, the differences between a 1080p and 720p image in a 50-inch projected image will be hard to spot, particularly if created with fine-pitch microdisplays such as LCoS and DLP. There are other factors to consider. A high-resolution image with image artifacts such as motion smearing, incorrect white balance or color points, and grayscale rendering problems may not look as realistic as a lower-resolution image without any of these problems. There have even been instances when higher-resolution displays have actually looked softer with HD content than lower-resolution models! In a recent test I conducted of CableCARD TVs, I had one wide VGA (852x480) plasma TV showing crisper-looking 720p and 1080i content than a native 1920x1080 LCD TV sitting nearby. I’ve also seen sharper HD pictures on those wide VGA plasmas than I have seen on higher-resolution 1024x1024 ALiS plasma monitors, probably due to the tricky scaling required to re-size the HD images to the non-square AliS pixel format. The argument for higher and higher resolution only makes sense when all other image parameters are set correctly. And we’ve also got problems in the transport stream and original content to overcome. Live 1080i programming at lower bit rates can come up short to live 720p material with higher bit rates, as macro blocks and mosquito noise all affect picture detail. In an ideal world, our 1080p content would be served up at a high bit rate (20 Mb/s or more) using MPEG-2 (or possibly more advanced codecs like H.264 or Windows Media 10) to a color-corrected, calibrated display with equal spectral response from the illuminating source and a nice, clean grayscale with no crushing of blacks or whites. The refresh rate would be either 24 Hz tripled to 72 Hz, or 60 Hz native – nothing lower. The signal interface from source to screen would be 100% digital to eliminate analog artifacts such as clipped bandwidth and ringing from standing waves. Many high-end home theater displays I’ve tested don’t have enough analog signal bandwidth to show even 720p HD signals. Does this type of display product exist? Well, there are some contenders for the throne, but you’d have to lay out from $120,000 to $250,000 to get one as they are all three-chip front DLP and LCoS cinema-grade projectors with xenon arc lamps and high bit rate HD-SDI interfaces. As for the consumer market, several ‘true’1920x1080 flat-panel LCD monitors and integrated TVs are available, but based on my recent tests of one new model, not all of the image parameters are lined up in a row yet. In this particular display, the grayscale rendering – while quite good – still isn’t wide enough, there are motion artifacts, and the HD signal bandwidth is again clipped. The point of all this is to show that a pile of pixels by itself doesn’t prove anything in a display. And beyond a certain viewing distance, the apparent differences between various displays with similar screen sizes and vastly different pixel counts are minimized, or disappear altogether if your eyesight is a little on the weak side. Still not convinced? Think about the resolution of the video and PC images you’ll be viewing most of the time. Having a ‘true’ 1920x1080 TV or front projector might do wonders for your ego, but if you plan to watch mostly 480i video and XGA or SXGA-resolution computer games on such a display, you’ll be less than thrilled with the resulting images. To sum up; unless you work for the Defense Mapping Agency, create computer graphics at Pixar, or have a 20-foot-wide screen in your home theater, projectors, TVs, and monitors with native Wide XGA (768p) or 720p resolution are still a good value and also ‘mature’ as a display category. Down-rezzed 1080i programming looks good at this resolution with a clean interlaced-to-progressive scan conversion. Similarly, it’s pretty simple these days to convert 480-line interlaced video to progressive scan and then scale it up to 1280x720, particularly with anamorphic (widescreen) programming. Of course, you can still dream on if you wish…………………. |
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