Wednesday, May 31, 2006

More Plasma Anomalies

As I said in an earlier post, my 32" Hitachi plasma can make black-and-white material look faintly greenish. That's not its only oddity, though. The amount of faux greenishness increases with decreasing settings of the color control (as I also reported). And reds tend to look orangish — not a true, satisfying red.

An article by HDTV expert Peter H. Putman, "The Plasma Doctor Is in the House," may explain this last anomaly. Putman says plasma panels, otherwise known in the trade simply as "glass," emit light when their constituent color phosphors are "tickled" by bursts of ultraviolet energy. The UV bursts come with an extra dollop of blue, UV's next-door neighbor in the color spectrum. To counteract the skew toward blue, plasma panels incorporate "capsulated color filters" that are designed in to restore a semblance of CRT-like hue.

"Other schemes have been tried to produce CRT-like phosphor response," Putman writes, "but the effects of UV color shift are still apparent with reds (they appear orange), greens (more of a lime green than a kelly or hunter green), and yellows (frequently shifting to a lemon, rather than an amber color)."

I'm not clear on whether the orangish reds come directly from the UV shift or from the wee color filters that are inserted in the light path to compensate for that shift. Whichever, it's clear that color on plasma TVs is a complex beast indeed. We're lucky it looks as much like CRT color as it does. (A CRT, for those not in the know, is a "cathode ray tube" — an old-fashioned picture tube, in other words.)

Putman also says that plasmas use "pulse-width modulation" to control how much light each phosphor emits: "a technique in which rapid on-off cycles can determine levels of luminance. The ratio of on cycles to off cycles within a given time interval translates into a specific luminance level."

(Here, "a given time interval" means a very tiny fraction of a second. Your eye can't actually see the on and off cycles, rest assured.)

Hence, says Putman, "on some panels, you may observe a color shift as brightness levels increase. The PWM method of simulating analog response works pretty well — my new GE electric range uses it to provide more control over the heating elements — but even PWM has its limits."

Translation: plasma display panels exhibit "non-linear response to changes in luminance levels. While a CRT is a purely linear display (small changes in driving voltage result in equivalent changes in anode current and brightness), a PDP is not." That's why it's apparently necessary for the user to lower the PDP's contrast setting below what the eye might ordinarily prefer. Otherwise, "you may observe a color shift as brightness levels increase."

I didn't mention it in my earlier article, but on my Hitachi plasma, the greenish tinge I see on B&W material seems to be more noticeable at high brightness levels than in relatively dark scenes.

None of this explains why my Hitachi's intrinsic grayscale calibration seems to vary with different settings of its color control — except to imply that with all the tweaking that is done in designing a plasma TV to get its hues even close to CRT-like, it's no surprise that there would be unsuspected interactions among the various user settings like contrast and color.

Another thing the Putman article reveals is that the process of calibrating a plasma's grayscale properly requires (a) a lot of hard-won expertise, compared with standard CRT calibrations, and (b) "a color analyzer with look-up tables for the specific phosphors used in each panel."

The specific-look-up-tables part seems to mean you can't use the generic look-up tables that normally come with a color analyzer, which is an instrument that objectively measures colored light sources.

"Chances are," writes Putman, "your panel came from one of these places: NEC, the Fujitsu-Hitachi plasma factory, Pioneer or Panasonic. (Although there aren't a lot of them out there yet, you will soon see panels coming from LG/Zenith and Samsung, and these will require their own phosphor look-up tables.) My FSR color analyzer is loaded with specific phosphor tables for each of the models listed (even the different phosphors in the Pioneer PDP-502 and PDP-503), thanks to Cliff Plavin of Progressive Labs, who took the individual measurements."

The expertise part comes in especially handy in performing the initial setup for the calibration process proper, in which you have to adjust the set's brightess and contrast controls to bypass the nonlinearities in light output spoken of earlier.

Both parts seem to suggest that, for us plasma TV owners, the idea of having our sets "professionally calibrated" — at no insignificant cost to us, I might add — may be fraught with danger. What if we happen to get a cocksure calibrator who's blissfully unaware of the pitfalls Putman has laid out? Or, what if the calibrator's ideas of proper brightness and contrast settings disagree with our own, such that when we readjust these variables after he leaves, his carefully metered grayscale goes totally kerflooey?

I'm not sure I'd even want a calibrator that doesn't have Cliff Plavin's home number in the directory on his cell phone, at any rate.

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