LCD Panels

Borrowing technology from laptop manufacturers, most major monitor makers sell monitors with liquid crystal displays (LCDs). LCDs have low-glare, completely flat screens and low power requirements (5 watts versus nearly 100 watts for an ordinary monitor). The color quality of an active-matrix LCD panel actually exceeds that of most CRT displays.

At this point, however, LCD screens usually are more limited in resolution than typical CRTs. For example, a typical 15'' LCD panel (which offers about the same viewable area as a 17'' CRT display) has a maximum resolution of 1024x768, whereas a typical 17'' CRT might offer a maximum resolution of 1280x1024 or 1600x1200.

17'' and 18'' LCD panels (comparable in viewing area to 19'' CRTs) have also become popular. However, these larger LCD panels offer a maximum resolution of 1280x1024, whereas a typical 19'' CRT has a maximum resolution of 1600x1200. Despite recent price drops, LCD panels continue to be more expensive than comparably sized CRTs.

A typical 15'' LCD display panel sells for around $250–$350, compared to flat-screen 17'' CRTs, which sell for around $150–$350. However, it is important to consider that an LCD screen provides a larger viewable image than a CRT monitor of the same size.

Two basic LCD choices are available today on notebook computers: active-matrix analog color and active-matrix digital—the latest development. Monochrome LCD displays are obsolete for PCs, although they remain popular for Palm and similar organizer devices and are sometimes used for industrial display panels.

Passive-matrix displays using dual-scan technology were popular for low-cost notebook models until recently, but most low-cost notebooks sold today use the brighter analog or digital active-matrix designs originally found on more expensive notebook computers.

Passive-matrix displays are still used with handheld organizers or for industrial-use desktop display panels because of their relatively low cost and enhanced durability compared to active-matrix models.

Desktop LCD panels are analog or digital active-matrix units. Typically, lower-cost 15'' LCD panels use the traditional analog VGA connector and must convert analog signals back into digital, whereas more expensive 15'' and most larger LCD panels provide both the analog VGA and the DVI digital connector found on many high-end and mid-range video cards.

Note that some LCD vendors of VGA/DVI-compatible panels might provide only the cheaper analog VGA cable, leaving it to you to buy your own DVI cable. If you plan to connect your new LCD display to your video card's DVI port, be sure that the panel supports DVI and that the cable is included.

And, while you're shopping for an LCD panel, be sure to note which models include the DVI cable; the presence of a DVI cable in the box of a nominally more expensive display can more than make up the difference in price between it and an apparently less expensive panel that doesn't include the cable.

How LCDs Work

In an LCD, a polarizing filter creates two separate light waves. The polarizing filter allows light waves that are aligned only with the filter to pass through. After passing through the polarizing filter, the remaining light waves are all aligned in the same direction. By aligning a second polarizing filter at a right angle to the first, all those waves are blocked.

By changing the angle of the second polarizing filter, the amount of light allowed to pass can be changed. It is the role of the liquid crystal cell to change the angle of polarization and control the amount of light that passes. The liquid crystals are rod-shaped molecules that flow like a liquid.

They enable light to pass straight through, but an electrical charge alters their orientations and the orientation of light passing through them. Although monochrome LCDs do not have color filters, they can have multiple cells per pixel for controlling shades of gray.

In a color LCD, an additional filter has three cells for each pixel—one each for displaying red, green, and blue—with a corresponding transistor for each cell. The red, green, and blue cells, which make up a pixel, are sometimes referred to as subpixels. The ability to control each cell individually has enabled Microsoft to develop a new method of improving LCD text quality. Beginning with Windows XP, you can enable a feature called ClearType through the Display properties sheet. However, individual cells can also fail.

Dead Pixels

A so-called dead pixel is one in which the red, green, or blue cell is stuck on or off. Failures in the on state are more common. In particular, those that fail when on are very noticeable on a dark background, such as bright red, green, or blue dots. Although even a few of these can be distracting, manufacturers vary in their warranty policies regarding how many dead pixels are required before you can get a replacement display.

Some vendors look at both the total number of dead pixels and their locations. Fortunately, improvements in manufacturing quality make it less and less likely that you will see a screen with dead pixels either on your desktop or in your notebook computer display. Although there is no normal way to repair bad pixels, there might be a simple fix that can help.

I have actually repaired bad pixels by gently tapping on the screen at the pixel location. This seems to work in many cases, especially in cases in which the pixel is always illuminated instead of dead (dark). Because I find a constantly lit pixel to be more irritating than one that is constantly dark, this fix has saved me a lot of aggravation.

Active-Matrix Displays

Most active-matrix displays use a thin film transistor (TFT) array. TFT is a method for packaging from one (monochrome) to three (RGB color) transistors per pixel within a flexible material that is the same size and shape as the display. Therefore, the transistors for each pixel lie directly behind the liquid crystal cells they control.

Two TFT manufacturing processes account for most of the active-matrix displays on the market today: hydrogenated amorphous silicon (a-Si) and low-temperature polysilicon (p-Si). These processes differ primarily in their costs.

At first, most TFT displays were manufactured using the a-Si process because it required lower temperatures (less than 400°C) than the p-Si process of the time. Now, lower-temperature p-Si manufacturing processes are making this method an economically viable alternative to a-Si.

To improve horizontal viewing angles in the latest LCD displays, some vendors have modified the classic TFT design. For example, Hitachi's in-plane switching (IPS) design—also known as STFT—aligns the individual cells of the LCD parallel to the glass, running the electric current through the sides of the cells and spinning the pixels to provide more even distribution of the image to the entire panel area.

Hitachi's Super-IPS technology also rearranges the liquid crystal molecules into a zig-zag pattern, rather than the typical row-column arrangement, to reduce color shift and improve color uniformity. The similar multidomain vertical alignment (MVA) technology developed by Fujitsu divides the screen into different regions and changes the angle of the regions.

Both Super-IPS and MVA provide a wider viewing angle than traditional TFT displays. Other companies have different names for the same technology—for example, Sharp calls it Ultra High Aperture (UHA). Manufacturers often like to think up their own buzzwords for the same technology because it makes their products seem different.

Because larger LCD displays (17'' and wider) are large enough to cause shifts in viewing angle even for an individual user, these advanced technologies are being used primarily on larger and more expensive panels and have been licensed to other display vendors.

Flat-Panel LCD Monitors

LCD desktop monitors, once seen mainly on the office sets of futuristic TV shows, are now becoming an increasingly reasonable choice for use in today's office computing environment. Many users with dual-display-capable video cards have added an LCD panel as a second monitor or use one as their only monitor.

LCD monitors offer a number of benefits when compared to conventional CRT "glass tube" monitors. LCD panels feature a larger effective viewable area than CRTs; a 17'' LCD is essentially equal in usability to a 19'' CRT. Because LCDs use direct addressing of the display (each pixel in the picture corresponds with a transistor), they produce a high-precision image.

LCDs can't have the common CRT display problems of pin-cushion, barrel distortion, or convergence errors (halos around the edges of onscreen objects). LCD panels are less expensive to operate because they feature lower power consumption and much less heat buildup than CRTs. Because LCD units lack a CRT, no concerns exist about electromagnetic VLF or ELF emissions.

LCD panels offer a smaller footprint (front-to-back dimensions), and some offer optional wall or stand mounting. Several LCD panels offer a pivoting feature, enabling the unit to swivel 90° and providing a choice between the traditional landscape horizontal mode for Web surfing and the portrait vertical mode for word processing and page-layout programs.

LCD panels weigh substantially less than comparably sized CRTs. For example, the ViewSonic VE175, a 17'' LCD display, weighs only 13.6 lbs., compared to the 50 lbs. weight of typical 19'' CRTs. There have been two major digital LCD display panel standards and specifications:

  • The Digital Flat Panel (DFP) standard approved by the Video Electronic Standards Association (VESA) in February 1999. DFP was previously known as PanelLink; DFP has now been replaced by DVI.

  • The Digital Visual Interface (DVI) standard proposed by the Digital Display Working Group (DDWG) in April 1999. DVI has become a de facto standard supported by most recent mid-range and high-end VGA display cards, including models with dual-display capabilities.

Figure below shows how DFP and DVI connectors found on some video cards and digital LCD displays compare to the standard VGA connector used on conventional video cards, CRTs, and analog-compatible LCD displays.

Before you rush to the store to purchase an LCD desktop monitor, you should consider several potential drawbacks:

  • If you routinely switch display resolutions (as Web developers do to preview their work), LCD monitors must take one of two approaches to change resolutions. Some older units might reduce the onscreen image to occupy only the pixels of the new resolution, thus using only a portion of a typical 1024x768 LCD panel to display a 640x480 image, whereas newer units might scale the image to occupy the entire screen.

Scaling is becoming more common because the Digital Display Work Group standard for LCD desktop displays specifies that the scaling must take place in the display panel, the graphics controller, or both places. Look at the quality of a scaled image if using different resolutions is important to you.

  • If you choose an analog LCD panel, you'll usually save money and be able to use your existing video card or onboard video port. However, image quality for both text and graphics can suffer because of the conversion of the computer's digital signal to analog (at the video card) and back to digital again (inside the LCD panel).

The conversion can lead to pixel jitter or pixel swim, in which adjacent LCD cells are turned on and off by the display's incapability to determine which cells should stay on and stay off. Most panels come with adjustment software to reduce this display-quality problem, but you might not be able to eliminate it entirely.

  • Digital LCD panels avoid conversion problems when attached to a digital-compatible display card. However, most low-cost off-the-shelf display cards don't support digital signals yet, and the onboard video circuits built into some motherboards don't support DVI yet.

  • High-quality LCD display panels of either digital or analog type are great for displaying sharp text and graphics. But they often can't display as wide a range of very light and very dark colors as CRTs can.

  • Many LCD displays don't react as quickly as CRTs. This can cause full-motion video, full-screen 3D games, and animation to look smeared onscreen. To avoid this problem, look for LCD displays that use an improved control method called feed forward driving (FFD) technology developed by Mitsubishi.

Thanks to price decreases, larger panel sizes, improved performance, and widespread support for DVI digital connectors on current video cards, this is the best time ever to consider buying an LCD panel for your desktop PC.

Be sure that you use the following criteria when you consider purchasing an LCD monitor:

  • Evaluate the panel both at its native resolution and at any other resolutions you plan to use.

  • If you're considering a digital LCD panel, determine whether your existing video card supports the features you need. Features you might find necessary include OpenGL and high-speed 3D support (for gaming), VGA-to-TV support (for video producers), and DVD playback software (for watching DVD movies).

Because most mid-range and high-end video cards based on the latest NVIDIA and ATI chipsets do offer a DVI port for connection with current and forthcoming digital LCD panels, you can upgrade to a high-performance video card that will support your display. Even though some notebook computers now support DVI displays, most still feature only analog VGA connectors.

  • Look for displays that support both analog and DVI inputs if you want to use the same display on different systems. Because LCD panels are much lighter and smaller than normal CRT displays, they're a natural choice for connecting to both desktop and notebook computers.

If you use multiple computers in a small work area, you might also want to look for displays that support multiple inputs, which enables you to connect two computers to one screen.

  • Make sure your system has a suitable expansion slot for the recommended video card type. Many low-cost systems today feature onboard AGP video but no AGP slot, which can't be upgraded unless the user opts for the obsolescent (for video) PCI slot.

As the move to LCD panels continues, more of these systems should feature built-in support for LCD displays, but this could be a problem for some time to come. NVIDIA's nForce2 and ATI's RADEON IGP integrated graphics chipsets support both DVI and analog VGA displays.

  • Evaluate the panel and card combo's performance on video clips and animation if you work with full-motion video, animated presentation programs, or games.

  • Although active-matrix (analog) and digital LCD monitors have much wider viewing areas than do passive-matrix and dual-scan LCD panels used in older notebook computers, their viewing angles are still usually much less than CRTs. This is an important consideration if you're planning to use your LCD monitor for group presentations.

To improve the horizontal viewing area, several vendors have developed patented improvements to the basic TFT display, such as Hitachi's in-plane switching (IPS), Fujitsu's multidomain vertical adjustment (MVA), and Mitsubishi's FFD—all of which have been licensed to other leading LCD display makers.

  • A high-contrast ratio (luminance difference between white and black) makes for sharper text and vivid colors. A typical CRT has a contrast ratio of about 245:1. Although LCD panels in the May 23, 2000 PC Magazine test had contrast ratios ranging from a low of 186:1 to a high of 370:1, newer LCD panels have even higher contrast ratios (up to 400:1). Panels could be viewed at an average horizontal angle of as much as 129° without loss of contrast.

  • Features such as integrated speakers and Universal Serial Bus (USB) hubs are pleasant additions, but your eyes should make the final decision about which panel is best for you. Because reviews of LCD panels often don't provide detailed analysis of horizontal and vertical viewing angles and contrast ratios, check display units in stores yourself. Be sure to view the displays from several angles. If you're adding the panel as a second display, be sure to check its off-axis image quality.