ProjectorCentral.com

Vertical banding and DLP rainbows

Subscribe to Receive Articles

LCD vertical banding or DLP rainbows?

Evan Powell, May 24, 2005
ProjectorCentral.com

If you are planning to spend in the range of $1,000 to, say $5,000 for a home theater projector, you have a basic choice to make: an LCD projector or a DLP projector. Each technology has advantages over the other, and neither is perfect. LCD and DLP both have the potential to produce undesirable artifacts in the picture. LCD can have vertical banding and DLP can produce rainbow artifacts. Here is how you manage these problems . . .

LCD vertical banding

On occasion, LCD projectors can manifest a problem known as vertical banding. Vertical banding consists of subtle (sometimes not so subtle) vertical bands often in the range of about 1-inch thick, evenly spaced across the entire picture. DLP projectors don't have this problem. Most LCD projectors don't either-it is not an automatic byproduct of the technology. It is a flaw that can occur in the manufacturing process of the LCD panels themselves, and some LCD panels will tend to manifest it more noticeably than others. The panels that cause the most visible problems are usually weeded out in the vendor's quality control process. However, those that create just a hint of banding often pass through QC, since it is prohibitively expensive to discard all parts that exhibit flaws which may have no practical impact on picture quality.

So, we end up with a situation in which some LCD projectors, even within a given production run on one model, will have no banding at all,

some may have a hint of it that does not rise to the level of a significant issue, and some may have a more visible problem that really does impair the enjoyment of the viewing experience. In the latter case, sometimes the problem can be mitigated with internal adjustments made by the dealer or vendor service departments, and sometimes the LCD panels or the entire unit need to be replaced.

As an example, we have two Panasonic AE700s in house. The older of the two was one of the first units built, and it is the unit we reviewed last fall. We saw some subtle vertical banding on that unit and reported it in the review. It produced some noticeable texture in gray screen test patterns, and in video on infrequent occasions depending on subject matter, but it was not considered to be a significant flaw. In December, Panasonic engineers came to visit, and on that occasion they called up the service menu and tweaked our AE700. The adjustments they made reduced the visibility of the banding on test patterns by about 50%. So a problem that wasn't a big problem to begin with was reduced, but not eliminated entirely, with internal adjustments.

We acquired a second AE700 about two months ago. This unit shows almost no banding at all on gray screen test patterns although it is still detectable. However it is even less visible than it was on our first unit after tweaking. On the second unit, banding is essentially zero with normal video material playing. When we detect it at all, it is because we are specifically looking for it.

Therefore, those who say they see no banding at all on their AE700 are correct, and those who say they've had a problem with banding on the AE700 are also correct. And this phenomenon can occur with any LCD projector. However, the good news is that since vendors are more in tune with the fact that home theater buyers really don't like vertical banding artifacts, we are seeing less of it these days than we did before.

You can manage your risk on the vertical banding issue by simply buying from a dealer with a consumer oriented return policy. Some dealers allow you ample time to receive and check out your unit, and if there are problems, not only with vertical banding, but with anything in the performance of the projector, you can return it for exchanges or refunds without restocking fees. If the unit you receive has a vertical banding problem, you will see it immediately. So it doesn't take long to check it out.

Always read the fine print on the dealer's Return Policy section of their website so you understand clearly the rules of the transaction. But the better Internet dealers, in particular the dealers who specialize in projectors, will enable you buy without worries of getting stuck with a unit that does not perform to your satisfaction.

DLP rainbow artifacts

DLP projectors have no vertical banding problems. But all of the inexpensive models (okay, say, those under $15,000) have a single DLP chip and a spinning color wheel that can produce visible color separation artifacts-commonly known as rainbows. People tend to perceive rainbow artifacts differently based upon how sensitive they are to them. But basically, they appear as a breakout of red, green, and blue color that shouldn't be there. When you see momentary splashes of color in Casablanca, you know something's not quite right. Not only are they visually distracting, but sequential color updating from the spinning color wheel can cause eye strain and headaches for many viewers. This problem never occurs on an LCD projector because LCD projectors don't use color wheels.

Some people are never bothered by DLP rainbows. I am one of them. I can watch a DLP projector all day long and never see visual distractions or develop eyestrain or headache. This is somewhat of a handicap for a projector reviewer. In years past I'd give a particular DLP projector a glowing review, only to get a flurry of email saying, "Evan, are you nuts? There are rainbows everywhere."

Individual sensitivity to rainbow artifacts can vary greatly. In addition to folks like me who don't see them at all, there are those who notice them from time to time, but don't find them to be much of a nuisance. On the other hand, there are some who are so bothered by them that they cannot watch a DLP projector under any circumstances.

In today's market, most business class DLP projectors have what is known as a 2x speed wheel. Many units designed for home entertainment have 2x wheels also. It is the 2x wheel that causes the most problems.

Texas Instruments and the manufacturers of DLP projectors have increased the speed of the sequential color updating in order to reduce the percentage of the population that is affected by rainbow artifacts. By using a different wheel design that uses two sets of red, green, and blue filters instead of one, the manufacturers are able to double the refresh rate on the screen. This wheel design is referred to as a 4x wheel, and it substantially reduces the number of people who are affected by rainbows. Our newest staff reviewer, Bill Livolsi, has trouble watching 2x DLP projectors for any length of time. But he has no problem at all with the 4x models.

These days we often stipulate in reviews of DLP projectors that rainbow artifacts are a potential hazard of the technology. Though the faster wheels speeds have substantially reduced the percentage of the population that is susceptible to rainbows, there are still those who find them a problem even at the fastest wheel speeds.

The bottom line is I personally would not use a DLP projector with anything less than a 4x wheel in my own theater. Even though I am not bothered by rainbows, I expect that others besides me will be using my theater, and I would not want the system that I am so proud of to be giving them headaches.

Furthermore, even entry level DLP projectors like the Optoma H31 and the InFocus Screenplay 4805 feature 4x wheels. These units are selling for a mere pittance these days. Now that street prices of these outstanding 4x DLP projectors have fallen to their current level, we no longer recommend any DLP projector with a 2x wheel for home theater applications.

Just as with the LCD vertical banding issue, you need to buy a DLP projector from a dealer that will grant you a favorable return and exchange policy. You must fire it up and audition it for at least two hours with everyone in your household who will be frequent users of the projector. The faster wheel speeds reduce but do not eliminate the possibility that you may experience visual distractions, eyestrain, or headaches. So give everyone a couple of hours of exposure to ensure that nobody is adversely affected. And you might try it with a classic B/W film like Treasures of the Sierra Madre or Casablanca for the ultimate test, since color artifacts, if you are prone to see them at all, will be most apparent on black and white material.

The bottom line is that LCD and DLP projectors can both exhibit flaws that impair the viewing experience. So be aware of the potential issues, and buy from dealers with favorable return policies and good reputations for customer service. By doing so you'll eliminate the risk of having LCD vertical banding or DLP rainbow problems on your home theater projector.

(04/20/19 - 05:27 AM PST)
Copyright © 1999-2019 ProjectorCentral.com All Rights Reserved.