Saturday, December 23, 2006

A New Bedroom HDTV for Me, Part VII

I've been complaining (most recently in A New Bedroom HDTV for Me, Part VI) about vertical striations (as I now intend to call them) in the picture on my new Sony KDL-40XBR2 1080p LCD TV.

Today I happened to look at the TV while it was off and noticed that its screen was reflecting glare from one of the windows in the room. There was a striation pattern in the reflected glare! I was able to capture it on my digital camera.

The glare-striations are not in the glare light itself. They stay still on the screen when I move my head slightly so as to reposition the glare "hot spot" with respect to the borders of the screen.

Although I can't really match the two banding patterns striation for striation, I'm convinced these two phenomena are actually one! That is, whatever causes the window glare to be non-uniform in its reflection from the screen is also what causes light from the LCD panel, when it is on, to be non-uniform in its distribution.

Why do I think this? Both patterns of non-uniformity involve vertical bands (striations) of various arbitrary widths. The widths are all fairly broad, on the order of an inch or so. They don't seem to appear on either end of the screen, just in the middle 50% or so. There are no horizontal striations, just vertical ones. The striations are quite faint in both cases. Their "edges" are indistinct and gradual in both.

The glare-striations are clearly not produced by the TV's electronics, since the TV is off. They are not due to backlight non-uniformity or leakage through the LCD panel, since there is no backlighting.

What exactly does produce them is hard to say. My best guess is that it's something physical, or mechanical, or geometric — whatever word you want to use for something not perfectly smooth and flat. Possibly the front, outer surface of the LCD panel is rippled. I notice that when I apply light finger pressure (using a tissue), the front surface gives a bit.

Or possibly the ripples are in a deeper layer of the LCD "sandwich."

An LCD panel in an HDTV is made of several layers, the first one (counting from the rear forward) being the backlight. The backlight simply produces light, which then passes through a vertical polarizing filter (the next layer) that polarizes it — makes sure all the light waves are aligned in one direction, the vertical one.

The comes the liquid crystal layer, sandwiched between two glass plates whose grooves cause the intervening sequence of liquid crystal molecules to be helically twisted in alignment. The light passing through the liquid crystal layer has its polarity twisted because the liquid crystal molecules are themselves thus arranged. So the light from the backlight will be able to pass right on through the next layer, a horizontal polarizing filter ... and the pixel will light up.

The color of the pixel — red, green, or blue — is produced by a tiny colored filter in the light path. The colored filters are yet another layer of the LCD sandwich.

The first glass plate in the sandwich houses an array of electrodes, one electrode for each pixel. Each electrode contains a transistor. If any given pixel's transistor has voltage applied to it, it will untwist the liquid crystal material in proportion to the applied voltage. The higher the voltage, the more the crystal untwists.

The result is that less of the light passing through the liquid crystal will be able to penetrate the horizontal polarizing filter. The pixel will darken.

If the maximum voltage is applied to the transistor for a given pixel, the liquid crystal will be fully untwisted, and the pixel will be black (or, at least, it will be as dark as it can be).

But if the liquid crystal "leaks," the pixel won't get that dark. It leaks when:
  • the applied voltage is too low
  • the appropriate maximum voltage doesn't fully untwist the liquid crystal
  • there are impurities in the liquid crystal
  • there are defects in the alignment of the various parts
  • etc.
If a lot of leaks are present in one area of the screen, clouds appear. There have been a lot of complaints about "cloudy" or uneven backlighting with this TV and its Sony siblings. See Official Sony 46" XBR LCD Uneven Backlight/Cloudy Thread for more on this. Luckily, I don't seem to have the cloudy backlighting problem. This is the problem I spoke of as "mura" in earlier posts.

The problem I have — vertical striations — could however be related to defects in the mechanical alignment of things on the various inner layers of the LCD sandwich, such that when light enters the panel from the front (as with the window glare) it is not reflected back evenly.

I have had not one but two LCD panels in my Sony: the original one and a replacement panel installed by an authorized technician after I complained of the striations. As far as I can tell, nothing whatsoever changed when the replacement panel was put in.

To my eye, that is, the exact same striations beset the second panel as beset the first. They appear to be in precisely the same locations on the screen. It is enough to make me wonder if they might be the result of physical irregularities in the front frame of the TV, against which the edges of the LCD panel are pressed when the necessary screws are tightened during assembly of the TV. If so, a replacement panel would be distorted in the same way as the original was distorted.

So that's where I stand on the striations issue at present. I am becoming convinced that there are mechanical distortions in the LCD panel which cause them to show up in pictures being produced on the screen and in glare reflected from the screen.


Anonymous said...

I have noticed this to happen as well. My question to you is, is this an LCD only glitch? Do plasma televisions also suffer from a problem like this? Also, have you noticed it on smaller LCD models as well or only the larger ones? I'm thinking that it may simply be a problem with such a large surface area (the screen) as oppossed to a problem with the LCD panel specifically, though I could be wrong. If it is the LCD panel, then again, I wonder if a plasma television would nip a problem like that in the bud. My wife and I are thinking of purchasing a new tv pretty soon, and one of the things we are still undecided on is whether to go with an LCD or Plasma model. Here's a guide that touches on some of those differences, but it doesn't mention anything about the glare problem that you are having. Any ideas? LCD only or are all big screen models susceptible to these kinds of things? Any info you could provide would be appreciated. Thanks!

eric said...

Dear Anonymous:

Plasmas do not have the mura/cloudiness problem. Neither do most LCDs. Even LCD models where some individual units are cloudy have other individual units that are fine.

Plasmas, though, are subject to burn-in, where LCDs are not. If any part of the image never changes or moves (think of a channel logo at bottom-right of the screen) it can get burned into the phosphors of a plasma. Since an LCD has no phosphors, that doesn't happen with an LCD.

I have a three-year-old Hitachi plasma in my basement, with the LCD in the bedroom. It has succeeded in resisting burn-in. But its colors are not as true as on the newer Sony LCD, reds being more of an orange hue. Also, black & white images take on a greenish hue on my plasma.

My Sony LCD has neither of those issues, and it has much better black levels and shadow detail.

Glare can actually be more of a problem on a plasma than on an LCD, since the former will have a more mirror-like front surface.

I don't know whether larger LCDs are more likely to have cloudiness/mura than smaller ones, since there is also the question of whether the real culprit is related to 1080p pixel resolution vs. 720p. The denser the pixel count per square inch, the more easily manufacturing flaws might creep in, I would think. So opting for a smaller 1080p LCD over a larger 720p might increase you risk.

I need to re-emphasize that the brightness non-uniformity that my Sony LCD has is not really a cloudiness issue, but more of a vertical striping. It seems to be a problem that does not crop up on many LCD sets. It is very hard to see the problem at all, virtually all the time. I consider it a much less obtrusive flaw than, say, the off-hues on my plasma.

I also have a three-year-old Samsung DLP set in my living room. It's not a flat panel, but a rear projector. It too has some issues with picture quality, particularly with what I think may be a non-uniform or irregular "gamma" curve. Darker scenes just don't look right on it. The Sony LCD beats it hands down in this regard. Also, the DLP seems to have trouble rendering nuanced facial gradations naturally.

The moral of the story is: all digital TV technologies have their weak points. Each year that goes by sees each technology get better and better. Still, you should not expect perfection with any TV you buy.

Try before you buy! You need to try out the TV you are thinking of buying in a dimly lit environment, with the picture settings backed off so you don't need sunglasses. In my area, Tweeter is the best bet for this, though Best Buy has one section where some of the more expensive TVs can be viewed in dim light.

You also need to watch a lot of stuff on your candidate TV purchase, particularly material in which the scene doesn't change every fraction of a second. Try out both high-def and standard-def material. Make sure you linger awhile over darker scenes. (But good luck in finding a store that will let you play, say, one of your favorite DVDs into any arbitrary set in the showroom.)

You also need to make sure that whatever TV you buy can be returned for full money back, no questions asked, within a stated period of time. If it's a flat panel and you've wall-mounted it, this can pose a problem, admittedly. The truth is, it is very, very, very difficult to dot all the I's and cross all the T's in any TV purchase these days.