LCD – For the Novice and the Expert
by Kristopher Kubicki on September 4, 2003 12:13 AM EST- Posted in
- Displays
Common LCD problems
What is ghosting?
Once upon a time, passive LCDs roamed the earth. Unfortunately, they were too slow and painful to look at for any extended period of time. 120ms response times assured anything over 7 frames per second left a lasting impression on your screen. Today, we have monitors that produce 16ms response times, although with the obvious sacrifice of color depth.Perhaps almost half of the people we talk to misinterpret ghosting. By definition from the VESA Flat Panel Display Manual, ghosting is actually the problem of interference over the signal, which results in an “echoed” image. However, misuse has somewhat skewed the word’s definition to also mean problems due to poor response times. To make matters more confusing, ghosting in the signal sense of the word and in the response sense are very hard to tell apart sometimes (especially if you don’t know what to look for).
We polled a few users in our forums that had “ghosting” problems. Three out of the five retracted their problem after we made them replace the analog signal cable with a DVI cable. Thus, we can infer that only two users really suffered from problems with the response time.
The new trend in LCD measurements is shifting to gray-to-gray response times, rather than black-to-white. As we mentioned earlier, black-to-white response times usually do not represent the most real world operations of LCDs. Part II of this LCD FAQ will deal most explicitly with response time issues, so we will not get too far into the subject here. As a simple word of advice, we encourage you to take a look at the LCD that you plan on buying in a store before you buy it online. Stay tuned for our Response Time follow up to this FAQ.
Other Image Problems
One of the other common types of interference on LCD screens has more to do with poor power supplies. Occasionally, an LCD will display an image, but seconds later, the image still persists. Below is an example:When the external brick overheats, we have noticed that its ability to produce clean signals degrades. Thus, the monitor does not properly modulate the signal. Our solution is to make sure your power supply is properly ventilated and not hot to the touch.
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spikemike - Thursday, September 4, 2003 - link
Just got the deal on passive displays from the LCD expert(my dad, he worked at RCA on LCDs just a couple years after they first invented it) and here is what he saidPassive displays do operate in either twisted or untwisted mode; data is supplied as on or off signal. However because the response time of STN is slow, they get gray scale by pulse width modulation techniques. Usually they get 16 shades of gray (4096 colors) with time modulation, and additonal gray levels by spatial modulation (using 2-4 pixels to achieve
1-2 bits additonal gray scale). Right now, the best STN color displays achieve 65K colors.
So basically if they need a gray scale they just turn it on and then turn it off before it gets all the way on. So technically the way it was written was correct. (by the way the last two posts were also mine)
Anonymous User - Thursday, September 4, 2003 - link
"Take a piece of lint free cloth and wrap it around your index finger. Push about 1/4 of an inch above the broken pixel with about as much force as would take to depress a doorbell. Pull your finger down past the pixel to about 1/4 of an inch below the broken pixel. Repeat in a left to right manner as well. If you’ve tried it about 10 times and are still without success, then you most likely are not going to be able to bring that pixel back to life (and you will probably have a light scuff mark on your anti-glare coating as well).">>>are you supposed to press the pixels while it's on? couldn't it cause the neighboring pixels to become stuck in the on position?
Anonymous User - Thursday, September 4, 2003 - link
I have a Cornea Mp704B 17" LCD. It has the 20ms Hyundai Panel. It's very nice, colors look great, sharp text and images, and no trailing or ghosting at all. I did notice that the backlight isn't perfect (just slightly, and i mean slightly, brighter in middle and bottom of LCD than on the top) and I did have 1 dead pixel and 2 dead subpixels (they weren't stuck on red though). I haven't tried rubbing, it sounds a little fisky, but I may eventually. But yeah, the Hitachi panels are nice overall.Anonymous User - Thursday, September 4, 2003 - link
#25 is right, passive displays are capable of color, every color cell phone today available in the US uses a passive STN display, as far as i know. It is still a much cheaper technology. As does gameboys and palm pilots. Thats why the color seems like pastels and not vivid colors.Anonymous User - Thursday, September 4, 2003 - link
i think 16 and 17 make a point, #16 almost all monitors use the 18bit color he is right about that. A 6 bit driver is much cheaper then an 8 bit driver. And maybe only a few very high end models use an 8 bit driver. #17 discusses the TV LCDs these in fact are the best LCDs you can buy, they have the widest viewing angles, best color, and the fastest response times, because all people do is watch video on them, Although they tend to be lower dpi so if you are thinking about getting one for a computer monitor be sure to sit far a way or you will be able to see the pixels. Some 60" LCDs shown at SID conferences are the best ever made. They look absolutely amazing.A third note is that high res high color displays are a requirement for longhorn. Sharp announced they would be the first to offer longhorn compatible displays. They need high res(UXGA probably) and high color(10bit i believe). These products are still about 2 years off but if you look at displays in laptops they have no problem making high DPI displays, for some reason companies don't think people want a UXGA 17" panels.(they should look at forums like this). There is also a company working on ways to get higher res out of the same number of drivers and subpixels by arranging the pixels differently, (www.clarivoyante.com) this could lead to lower cost for the same resolution potentially allowing the better substrates to be used. There are substrates that use high response time, wide viewing angle technology, its just considerably more expensive.
Anonymous User - Thursday, September 4, 2003 - link
#14No I, run my CRt at 85 hz .Well that's as far as my card will go .old card TNT2
Anonymous User - Thursday, September 4, 2003 - link
No.I run it at 85 Hz.Well that's the maximum my card will go .OLd card TNT2.Live - Thursday, September 4, 2003 - link
Very Good article! Anandtech is really getting back in good shape. I for one would like to read one about CRTs as well. While you’re at it why not cover monitors in your price guides as well what’s the use of all this computer power and fancy graphics cards without a good output of it all?Anonymous User - Thursday, September 4, 2003 - link
The parts about passive and TFT technology is confusing and perhaps misleading. If you're talking about passive matrix STN or DSTN LCD displays, they were indeed able to turn a pixel partially on, or partially off. However, they're biggest downside was they were always fuzzy because manipulating one pixel, often caused the ones surrounding it to discolour, too. The biggest benefit from TFTs, I believe, was that each individual subpixel was uniquely addressable.Anonymous User - Thursday, September 4, 2003 - link
samsung still not offering up their 'X' line of 16ms panels for review? Apparently they are available for sale in Europe...