A Guide to LCDs

[mista_haqeem]

This information started in another thread post, and I decided to make a new thread at the request of others. I hope that it is helpful to you. If anyone finds any errors or would like to expand, please feel free to do so!

There is a basic relationship in LCD monitors that is often overlooked. Simply stated, image quality is inversely proportional to the refresh rate of a monitor. In other words, the faster the refresh rate, the poorer the image quality. I am, of course, generalizing and there are plenty of exceptions.

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| 6-bit versus 8-bit panels |
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Lets start with color representation. There are lots of ways to represent colors in a numerical format. The common is to use rgb method. This is defined as a 32-bit number like so: 00RRGGBB where the first 8 bits are 0's (unused), the next 8 bits are for red, the next 8 bits are for green, and the last 8 bits are for blue. This is called 24-bit color (note: windows calls it 32-bit color because values are stored in 32-bit integers). You set this value in color quality section of the display properties in windows. Note: you can actually use true 32-bit color, although windows and most consumer level graphics cards/monitors don't support it. YOu need some high end unix stuff from someone like SGI for that.

So, how does this relate to LCD's? LCD's also use a numerical format to represent numbers, although their naming convention is different. There are two types of displays available, 6-bit displays and 8-bit displays. This refers to the number of bits per color. In other words, an 8-bit panel corrosponds to 24-bit color directly and 6-bit color refers to something not available in windows (although it's close to 16-bit color). What this means is that even though you may set the color depth to 24-bit (it reads 32-bit remember), you are not seeing that bit-depth. Now how much of an impact does this have? 8-bit displays can display a total of 16.7 million colors. 6-bit displays can only display 262 thousand colors. That's almost 100 times more colors on the 8-bit display.

Now of course showing that few colors is completely unacceptable, so LCD manufacturers employ a little trick called dithering. It works by tricking you into seeing a color that isn't really there (it's rather clever actually). If it has to display a non native color, it takes two adjacent pixels and sets them to values just to either side of the desired value. The result is that the eye thinks it sees the proper color, even though it really doesn't. Printed media that uses only 4 colors of ink but places two colors right next to each other works the same way essentially (think magazines and newspapers). However, even using this method it can still only "display" 16.2 million colors. As a side note: you can tell if a panel is an 8-bit or a 6-bit panel by checking if it can display 16.2 million colors or 16.7 million colors. Newegg reports all panels in this manner. Ok...now why is this bad? Depending on how sensitive your eyes are, you can see the dithering taking place. I usually see it as a sort of "banding" usually, and the colors don't quite look as good. It's like the old refresh of CRT monitors...some people are perfectly happy with 60hz, and some can't stand 75hz. In any case, the fact is that the images you are seeing on a 6-bit panel are not completely accurate. In an 8-bit panel, images look very rich in color without having to use any extra features such as increasing the contrast too much or using something like digital vibrance in the nvidia drivers. Color representation is pretty much perfect on an 8-bit panel (except in really dark shades due to bleed-through of the backlight, which all TFT panels have).

So why are 6-bit panels so common? They are a lot cheaper to make, and they have faster response times. This is why we saw a jump in refresh times from the 18ms range to the 8ms range almost overnight when the 6-bit panels were first introduced. And of course consider that companies have been stressing response times for years since they were so much of a problem at first, that pretty much everyone viewed response time as the only criteria in selecting an LCD. Companies capitalized on this, and that's why most panels today are 6-bit panels unfortunately.

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| Common Parameters |
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There are many different parameters that characterize an LCD. The order of importance varies from person to person and their needs, and ultimately you have to decide for yourself which parameters are most important.

Dot Pitch - the size of the individual pixels. The larger this is, the more blocky images look

Image Brightness, aka Black level - typically they mean the maximum brightness, measured
in candellas. You don't always want more. The problem with LCDs is that the back light stays on all the time, and an LCD filter is used to block the light to make black. These filters are never perfect and so a little light leaks through (called bleed-through). The brighter a display is, the greater the potential for bleed-through. So you need to take that into consideration.

Contrast ratio is the difference between pure white and pure black - This helps define the dynamic range of the monitor, but there are better methods.

Delta E (or delta tracking) - the short of it is that it compares the expected color to the displayed color...you want this as close to 0 as possible across all values

Contrast Stability - how steady is the contrast across multiple brightness values

Spacial uniformity - how uniform is the brightness between pixels. Usually the edges are darker than the center.

Related to the delta e (and i forget the term) is how the recorded versus expected values across all three colors plot. Some colors may be more off than others. If you set each color to the same value from 0 to 255 you should get a black/gray/white color. However, not all colors are the same and so they will have a slight tint to them. You want the graph of the brightness of all the colors to lie directly on top of each other. This goes back to delta e...the bigger the delta e, the bigger the problem of color alignment.

That's all I can think of off of the top of my head. There are other intersting parameters out there. also note: you can't get most of the values from the manufacturer, you have to get them from a 3rd party test site (like anandtech or tom's hardware).

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| Overdrive |
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There is a technology called overdrive. Basically, the way an LCD works is that you have a white backlight, and then you have color LCD filters on top of it. The filters are engaged by applying an analog voltage to it. The higher the voltage, the bigger the change in the filter. Now one side effect is that the higher the voltage, the faster the filter responds (thus the faster the refresh rate). A technique was developed at viewsonic first and is now available with many manufacturers that essentially "overshoots", i.e. it sets the voltage higher than the desired value and then backs it off as the color approaches the correct value. It works quite well, but if it is not implemented correctly you can get incorrect color values.

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| Recommendations |
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Ultimately you need to find an LCD with characteristics that match your personal needs. A graphic artist needs a display that is vastly different than a gamer. In my opinion, Viewsonic makes very good LCDs. I have the VP930b which is overall a very good display. It has very good color representation, although suffers from bleed-through, especially in the four corners resulting in a slight X shape. It only affects very dark colors though, and so it's livable. I did a significant amount of research for a monitor and settled on this one because it is a good general purpose monitor. It doesn't have any standout qualities (other than that it is an 8-bit panel), but it also doesn't have any weak areas.

A friend of mine has the Viewsonic VX922 is a cheaper 6-bit panel and is excellent for gaming, although I wouldn't recommend it if you do any sort of graphics work as it is a 6-bit panel and doesn't have very good color representation IMO. I used to have an Apple Cinema display where I worked and they have very good image quality, but have a slower refresh rate and are not suited for gamers. The colors were just gorgeous, but suffered from visible ghosting.



Finally, make sure that you use a DVI connection whenever possible. As much as I've tried, I've never been able to get the color balance as good on a VGA connection as I have on a DVI connection when identical monitors in a dual monitor configuration.


TFT Technology Breakdown and Model/Panel Index
LCD Panel Technology Type and Characteristics

TN film (Twisted Nematic)
- low manufacturing/retail costs
- restrictive viewing angles
- fast pixel response times
- dead pixels display white. Stuck pixels display RGB colors
- lower contrast levels means blacks are not as dark as VA based panels
- lower color reproduction

IPS (In Plane Switching)
- improved viewing angles over TN
- very good color reproduction
- slower pixel response times than TN
- dead pixels display black
- lower contrast levels means blacks are not as dark as VA based panels

Super-IPS (S-IPS)
- same as IPS except ...
- likely best color reproduction of all TFT
- less expensive to produce than IPS
- improved pixel response


VA (Vertical Alignment) Technologies

MVA (Multidomain Vertical Alignment)
- compromise between TN and IPS technologies
- superior color reproduction over TN but not as good as IPS
- very good viewing angles but less than IPS
- higher contrast than TN or S-IPS means very good blacks
- dead pixels are black
- slower pixel response than TN or IPS
- details can be lost when directly viewing dark areas

Premium-MVA (P-MVA)
- same as MVA except ...
- "overdrive" technology increases pixel response but still slower than TN
- may have slightly degraded color reproduction due to "overdrive" process

PVA (Patterned VA)
- same as MVA except ...
- larger viewing angles
- higher contrast levels means darkest blacks

Super-PVA (S-PVA)
- same as PVA except ...
- “Magic Speed” (the Samsung equivalent to Overdrive) improves pixel response
- slightly improved color reproduction
- slightly improved viewing angles



Purchasing Considerations

TN Gamers
Considered a "gamers" panel due to it's fast pixel response times which reduces trailing images know as "ghosting". However, this advantage has been reduced by new technologies to accelerate pixel response times in other panel types. Colors and contrast tend to be weak and blacks are not truly dark. Viewing angles are significantly limited. However, monitors based on this technology tend to be inexpensive.

IPS / S-IPS Graphics Work or Web Browsing
Considered to have the best color reproduction of all panel types, these panels are well suited for graphics work or web browsing. Pixel response time is also good but slower than the TN "gamers" panel. Contast and blacks are also less dark than VA panels but viewing angles are excellent.

MVA / P-MVA / PVA / S-PVA Compromise for All-Around Use
These panels are a compromise between the fast pixel response times of the TN panel and the excellent color reproduction of the IPS panels. Contrast and blacks are best of all the panel types. Viewing angles are similar but slightly inferior to IPS.


Model/Panel Index


Acer
Acer AL1951Cs - 19" TN Film
Acer AL1916ws - 19" WS TN Film
Acer AL2016ws - 20" WS Chunghwa Picture Tubes TN Film
Acer AL2032wa - 20" WS AU Optronics P-MVA (M201EW01 V0) OR LG.Philips S-IPS (LM201W01)

BenQ
BenQ FP202W - 20" WS Chunghwa Picture Tubes TN Film

Dell
Dell 1905FP - 19" Samsung PVA (LTM190E4-L02) or AU Optronics P-MVA (M190EN03 V0)
Dell 1907FP - 19" TN Film
Dell 2001FP - 20" LG.Philips S-IPS (LM201U04)
Dell 2005FPW - 20" WS LG.Philips S-IPS (LM201W01)
Dell 2007WFP - 20" WS LG.Philips S-IPS (LM201W01)
Dell 2007FP - 20" LG.Philips S-IPS (LM201U04)
Dell 2405FPW - 24" WS Samsung PVA (LTM240M1-L01)
Dell 2407WFP - 24" WS Samsung S-PVA (LM201W01)
Dell 3007WFP - 30" WS LG.Philips S-IPS (LM300W01)

Gateway
Gateway FPD2185W - 21" WS Samsung S-PVA (LTM210M2)

HP
HP F2105 - 21" WS Samsung S-PVA (LTM210M2)

Hyundai
Hyundai L90D+ - 19" Samsung TN (LTM190EX-L01)

NEC
NEC LCD20WGX2 - 20" LG.Philips AS-IPS (LM201W01)

Sceptre X20G Naga II 16ms 20" P-MVA
Sceptre X20G Naga II - 20" WS P-MVA

Samsung
Samsung 930B - 19" Samsung TN Film
Samsung 940B - 19" Samsung TN Film
Samsung 940T - 19" Samsung PVA (LTM190E4)
Samsung 940MW - 19"WS Samsung TN Film (LTM190M2)
Samsung 204B - 20" Samsung TN Film (LTM201UX)
Samsung 215TW - 21" WS Samsung S-PVA (LTM210M2)
Samsung 244T - 24" WS Samsung S-PVA (LTM240M2)

ViewSonic
ViewSonic VA1912W - 19" WS Chi Mei Optoelectronics TN Film (M190A1)
ViewSonic VP912B - 19" AU Optronics TN Film (M190EN04 V1)
Viewsonic VX922 - 19" AU Optronics TN Film (M190EN04)
Viewsonic VX924 - 19" AU Optronics TN Film (M190EN04 V5)
ViewSonic VP920 - 19" TN Film
ViewSonic VP930 - 19" AU Optronics P-MVA (M190EG01 V0)
ViewSonic VA1912W - 19" WS Chi Mei Optoelectronics TN Film (M190A1)
ViewSonic VA2012WB - 20" WS Chunghwa Picture Tubes TN Film
ViewSonic VX2025WM - 20" WS AU Optronics P-MVA (M201EW01 V0)

[PsYkHoTiK]

Please post credits when you copy paste stuff...

[Max_87]

Yeah, at least post the link to where you found this.