Monday, June 13, 2005

HDTV Quirks, Part I, Grayscale

In Picking an HDTV I gave a rundown on some of the basics of choosing an HDTV. Now I'd like to start pointing out some of the quirks I've run into with my own two HDTVs, in hopes that they may prove instructive to other HDTV buyers.

My first HDTV is a 61" Samsung DLP-based rear projector with native 1,280 x 720p resolution. My second HDTV is a Hitachi 32" plasma. The Hitachi's resolution involves a 1,024 x 852 grid of pixels — greater than the Samsung's resolution vertically but less horizontally.

These two TVs share a common flaw. The nature of this fault is alluded to in a review of another, newer Hitachi plasma in the March/April 2005 The Perfect Vision. Randy Tomlinson writes in his review of the Hitachi 42HDX61 42" Ultravision (pp. 74-76), "Various older black-and-white movies on some of VOOM's Original movie channels were shown without a color tint — which is very rare."

Neither of my HDTVs can make the same boast. Black-and-white material on both of them usually takes on a slight greeish cast.

On the Hitachi, furthermore, how green the cast is depends on the setting of the Color control. When Color is set moderately high, there's less green in a black-and-white picture, oddly, than there is when Color is reduced to lower and lower values ultimately approaching zero. (But when Color is reduced all the way to zero, the greenish tinge completely disappears!)

The green cast on the Samsung is independent of the Color control. However, it's strongest on composite video and S-video inputs and almost absent on component video an DVI.

Selecting different color temperature settings on the respective sets makes little difference, except that in some cases the greenishness may be partly masked by a notable reddishness or bluishness, depending on the color temperature selected.

I take it that the greenishness is associated with the sets' grayscales. "Grayscale" is geek-talk for how precisely the red, green, and blue primary colors are balanced into a neutral gray. This optimum grayscale balance has to exist across the entire range of shades of gray between pure black and pure white.

Exactly which version of neutral gray the set is ostensibly balanced for depends on the color temperature setting the user selects. "Cool" settings produce bluer grays, while "warm" settings give redder grays. Supposedly, the "correct" color temperature setting produces a very slightly reddish gray; this color temperature is the one geeks call "D6500" or "6500° Kelvin."

In all this "science" of color temperature, there's no real provision for opting for greener or less-green grays. The reason is that the various "legitimate" color temperatures lie along a well-defined curve on a piece of graph paper. For this particular curve, the green component of gray is always cast in concrete. If a black-and-white image on a real-world TV gives grays that are too green (or not green enough), it's because the set is "calibrated" to a point on the graph paper that doesn't lie on the approved curve.

As a result, not only will black-and-white images look green around the gills, but color images will not be exactly as they are supposed to be.

Calibration is the key word here. As it comes from the factory, almost any model of HDTV will not be calibrated to give a uniform, spot-on-D6500, not-at-all-greenish grayscale at every brightness level that is capable of being displayed.

The reasons why these sets are not "properly" calibrated at the factory are murky. One of them seems to be that these sets are actually miscalibrated to look good under harsh, bright fluourescent lighting in video stores. In such environs, the sets have to be "cranked" to maximize light output. That usually means that one or two of the three primaries will "run out of gas" before the third does. Green, for instance, often doesn't have the dynamic "legs" of blue ... and red is even worse.

So green may be artificially boosted in the underlying grayscale to compensate for the difference between the most green and the most blue the set can crank out. Meanwhile, red is typically boosted in the circuits that decode an actual full-color signal, thus keeping flesh tones right. This boost to red is what geeks call "red push."

Hence, it's up to the finicky user to have the set re-calibrated. That usually involves paying a technician certified by the Imaging Science Foundation (ISF) to bring weird instruments into your home and use them to guide tweaks he or she will make to numeric parameters in the TV's hidden "service menu."

I haven't had my sets ISF-calibrated. I tried — oh, yes, indeed, I tried. I found a local TV repair outfit which offers the service. I called on the telephone for an in-home consultation. A professional phone-answering person was my contact. She wouldn't let me speak to the actual technician, nor leave a message to be called back by him personally. I was instead told to let my new Samsung DLP "settle in" for at least three months, in order that the calibration, when done, would be a proper and permanent one.

Fine, said I. But can I make an appointment in advance for a date three months from now?

No, I was told, just call back when the time comes. Which is what I did ... only to be told that (a) the ISF tech does his calibrations on Saturdays only, never on weekdays, and that (b) he was at that time all booked up for the coming eight weeks.

So I put my name on his queue, waited eight weeks, and at the very last minute was called by the professional phone-answerer to say that the tech had fallen ill and regrettably needed to take the weekend off.

I insisted on talking to the guy (for the first and only time). He was really sick, when he at long last called me back, so much so that his voice was a mere croak. I felt sorry for him ... but when I mentioned how long I'd been waiting for his services, and how much I really didn't want to go back on the very end of his 8-week-long queue, and how I thought he might make an exception in my case and do my calibration on a weekday — after he got well, of course — he stiffed me.

That was the last contact I had with him or his company.

Then I found a guy online who claimed he could walk me through self-calibrating my Samsung remotely. We went through a number of iterations in which he fed me several bunches of service-menu changes he'd successfully used on Samsung DLP's he'd actually calibrated in person.

Trouble was, none of these sets of changes made my picture any better. For the most part, they made it worse. I suspect the fact that he conspicuously wasn't succeeding in getting my TV dialed in aright eventually took a toll on his ego. After a while, he stopped answering my e-mails.

Having had such lousy luck getting the Samsung calibrated, I never even tried with the Hitachi.

There is, I understand, no guarantee that even the best ISF calibration will produce a perfectly neutral grayscale, anyway. Not with these newfangled fixed-pixel, non-CRT, "digital" displays.

In the old days, a TV with a color picture tube didn't really have to work all that hard to produce a uniformly neutral grayscale. The light-emitting phosphors were, after all, of fairly standard hues, red, green, and blue. If the circuitry driving the CRT was set up properly at one image brightness or "IRE" level, chances were all other IRE levels would fall right in line.

The circuitry was not digital, but analog. Furthermore, it did not have to do weird things to the input signal to compensate for the inherent non-CRT-like characteristics of the display. After all, it was a CRT, through and through.

Not so, plasma. Not so, LCD, DLP, LCoS, DILA, and all the other arcane letters in the digital display alphabet soup. These fixed-pixel digital devices typically don't have phosphors of the same exact colors as a CRT's (in the case of plasma), or they don't have phosphors at all (in all the other cases). Additionally, their operating characteristics aren't at all CRT-like. So they have to digitally process the input signal to hide these facts.

The digital signal processing (DSP) functions they use to do this are not perfect. Compromises are required. It's simply a matter of moving the compromises around to where they're least bothersome to the viewer ... which is why there are parameters that can be tweaked in the TVs' service menus.

But with the most skilled tweaking by highly trained geeks with extremely expensive and sensitive instruments, results typically vary. I recently read in the "Technical Forum" portion of the January/February 2005 The Perfect Vision (p. 16) this response to a reader's question by the same well-versed Mr. Tomlinson:
[The] excessive green [in the ISF-calibrated picture] could be coming from two places. First, the ISF technician's color analyzer could have been inaccurate, and even the slightest errors toward green are instantly picked up by the eye. ... The second possibility lies in the color decoder adjustment. If that's done [by the technician] using color filters (the way we usually do it), there will be a green push to the picture. This green [unlike the first kind] will not show up on a black and white picture ... .
So color analyzers costing up to $15,000 can be inaccurate, says Tomlinson, as can the color filters used to gauge "green push." What are we paying these guys for, if the results do not necessarily come out spot-on accurate? Inaccurate is what we went in with!

I'm not saying fixed-pixel HDTVs are crap, or that ISF calibrations aren't worth it. I'm just saying that in this life we should expect to have to make tradeoffs. When we buy a fixed-pixel, high-tech, exotic HDTV, we should realize going in that they're not perfect. They may be really, really bright. They may have dazzling colors. They may be capable of stunning video resolution. But they have Achilles' heels.

For instance, few of them can render deep, satisfying blacks, or nuanced shadow detail. Many of them are guilty of "false contouring," where shades and colors that are supposed to grade smoothly into one another appear to have ersatz discrete bands. And most of them, apparently, have grayscales that, come hell or high water, are slightly tinted green or some other hue.

Some things you just have to live with.

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