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Plasma in the Digital Studio
Posted Aug 12, 2003 Print Version     « previous Page 2of 3 next »
  

Maybe they made that claim for historical reasons. Plasma manufacturers like Hitachi used to claim a "pre-filter" brightness—without telling people what it was. At least Hitachi now shows both—a pre-filter brightness of 750 nits and a post-filter (which is the way the plasma set arrives at your studio) brightness of 350 nits for their 4121HDU. The difference between 750 and 350 is the light absorbed by the anti-glare, anti-EMI, contrast-enhancing filter applied to the front surface of the plasma glass sandwich at the factory. That filter only lets about 46% of the light go through, trapping the rest. So if I multiply Sampo's 500 peak nits by 46%, then I'm left with about 230 nits "post filter," and that's probably what you'd find on a Sampo display if you turned both the contrast and brightness up so far that the image was totally washed out. Great specs, huh? Maybe that's why NEC doesn't make those kinds of claims at all: they don't want to participate in such games.

The same kind of logic is generally applied to the contrast ratio. Take some unrealistic brightness spec found only in the lab and divide that by an unrealistic black level and you'll get a number seen only by scientists looking through microscopes. I'd rather do it myself. When I used the ANSI 16-point "checkerboard" contrast pattern I found that the Sampo's contrast was only 123.93:1—a far cry from their claimed 580:1. I also measured the contrast using full screen on and off patterns with only 33% APL settings to get a better idea of "real world" video contrast—and under those conditions, I found only 254.46:1 for contrast ratio. That's a little higher than the Hitachi's reading under the same condition because the Sampo unit gains brightness with lowered picture level faster than the Hitachi does—however, the Sampo's total black level was about 33% higher than the Hitachi's and 42% brighter than the NEC 42MP4's low reading. That high black level (actually a dark gray was the best that the unit could do) results in an overall lower contrast level.

Anytime I looked at the Sampo unit side by side with another plasma showing the same image, the Sampo's looked a little washed out from its lower contrast ratio. Regular readers know that I've used the Mars Attacks DVD for several years to do side-by-side comparisons of display products, and that DVD easily showed a few faults. The Sampo plasma, for example, couldn't show all of the stars in the star field in the opening credits space shot. If you don't have the contrast, you're going to lose some of the white stars in the too-bright darkness of space. The other plasmas didn't make this mistake. Along with a lack of contrast, the Sampo was missing a lot of blue color saturation. If Sampo could fix those two problems and crank up their video processing circuits, while keeping their low price, I'd be a lot happier—in spite of their wild claims.

Hitachi CMP 4121HDU: Green in Judgment
Hitachi's been selling their 1024x1024 resolution plasmas for a couple of years now. The one I tested was the recent version of this series and my test results show that the main difference between today's unit and those first shown is a lower price. Hitachi has cut the price for this unit over the years to about one-half of what it used to be. And that's a great deal, all things considered, because the Hitachi 4121 HDU can now be found on the Web for about $4,800—again, close to $100 per diagonal inch, like the Sampo. However, each dollar spent on the Hitachi plasma buys about twice as many RGB pixels as you get on the Sampo.

I'll gladly take more pixels any day. Not only will the resulting image look better with any computer input, more pixels also do a better job with video as long as they can keep the brightness up while using smaller pixels. However, brightness is not a problem with the Hitachi (as long as you ignore their spec). The Hitachi 4121 gave close to the same 10% peak brightness as the Sampo unit did: 207 nits (while burning 355 watts of fossil fuel with a full screen). What's more, the Hitachi does that with a higher ANSI checkerboard contrast ratio, 160.36:1. Overall, the Hitachi showed better image quality than the Sampo did along with much higher resolution.

The only things I want to pick on Hitachi for (besides the wild pre- and post-filter brightness specs which they'll never live up to) are slightly lower corner brightness and slightly too much green color saturation. Most plasmas have corners that are every bit as bright as are the centers of the plasma screen. And that kind of image quality is very cool I'd say, since the result is an image with almost photographic flatness. All of the best monitors strive to have bright corners and most do. However, the Hitachi unit measured only about 95% as bright in the corners as in the center. Most people probably won't even see the difference between 99% and 95%, but if you're using your plasma for preview, you want to see your project represented as accurately as possible on screen before you go back to tweak what may or may not need tweaking.

The key issue in the image a plasma screen presents is color fidelity, and in this regard, most people can easily tell the difference between images with too much green and too little blue in comparison with perfectly colored images. The Hitachi unit had too much green color saturation, and needed to be re-adjusted to make decently realistic images. The Sampo unit had too little blue color saturation, leaving out part of the image. Now, I'll take too much green over too little blue any day since you can always turn the green color down (which darkens the overall image). But you can't put the blue back in if it never existed in the display device.

In case you were wondering, since plasma technology is based upon the same physical principles that power your fluorescent desk lamp, a lot of the hard work expended in developing plasma displays has been done to get good, balanced colors. Desk lamp light (and lumens or nits) is mostly green with only little bits of red and blue thrown in for balance. Consequently, the phosphors used in desk-lamps and plasmas are tilted towards green efficiency. If you can make green light efficiently, then you're most of the way done. However, video—whether HD or SD—requires the use of red and blue, too, and that's where I like to make critical judgments. I'll take a color-balanced plasma over one that's too green or one that's lacking in red and blue because colors are where it's at.


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