LCD Displays

LCD Displays

LCD displays use a relatively new technology, but all of the early teething problems have long been worked out and the prices of LCD displays have fallen to the point that they are now mainstream products. A good LCD display, such as the ViewSonic VP191 19" model shown in Figure 11-2, provides top-notch image quality in a compact package. Although traditional CRTs have advantages of their own, most people who experience the bright, contrasty image of a good LCD display will never return to using a CRT monitor.

Figure 11-2: ViewSonic VP191 19" LCD display (image reprinted with permission from ViewSonic Corporation)

WHY ARE THOSE STRAIGHT LINES CURVED?

If you convert from a standard CRT display to a flat-screen CRT display or (particularly) an LCD display, you may notice an odd effect. Your eye and brain become used to seeing the curved surface of the old display as flat. The new display, which truly is flat, looks concave! Straight lines appear to bow inward, particularly if you work close to the display. The effect is so convincing that Robert actually held a straight-edge up to his new LCD display. Sure enough, the "bent" lines were straight. Don't worry, though. The optical illusion disappears after only a couple hours' use.

CRT monitors were the dominant PC display technology until recently, but that has changed. For displays bundled with new PCs, LCDs exceeded CRTs in popularity by late 2002. By 2005, LCDs had also begun to outsell CRTs in retail channels. Lower cost and other advantages of CRTs ensure that they'll remain available for years to come, but the emphasis has definitely shifted to LCDs.

LCD display characteristics

Here are the important characteristics of LCDs:

Resolution

Unlike CRT monitors, which have a maximum resolution but can easily be run at lower resolutions, LCDs are designed to operate at one resolution, called the native resolution. You can run an LCD at lower than native resolution, but that results in either the image occupying only part of the screen at full image quality or, via pixel extrapolation, the image occupying the full screen area but with greatly reduced image quality.

Interface

LCDs are available in analog-only, digital/analog hybrid, and digital-only interfaces. Using an analog interface requires converting the video signal from digital to analog inside the PC and then from analog to digital inside the monitor, which reduces image quality, particularly at higher resolutions. Synchronization problems occur frequently with analog interfaces, and can cause various undesirable display problems. Finally, analog interfaces are inherently noisier than digital interfaces, which causes subtle variations in display quality that can be quite disconcerting.

Refresh rate

Whereas CRT monitors require high vertical refresh rates to ensure stable images, LCDs, because of their differing display technology, can use much lower refresh rates. For example, at 1280x1024 resolution on a CRT monitor, you'll probably want to use an 85 Hz or higher refresh rate for good image quality. At the same resolution on an LCD, 60 Hz is a perfectly adequate refresh rate. In fact, on LCDs, a lower refresh rate often provides a better image than a higher refresh rate.

Response time

Unlike CRT monitors, whose phosphor-based pixels respond essentially instantaneously to the electron beam, LCD panels use transistors, which require time to turn on or turn off. That means there is a measurable lag between when a transistor is switched on or off and when the associated pixel changes to the proper state. That lag, called rise time for when the transistor is switched on and fall time for when it is switched off, results in a corresponding lag in image display.

Everything You Always Wanted to Know About LCD Response Time

Fast LCD response time is a Good Thing. Fast response means smoother scrolling and no ghosting or smearing, even when you view fast-motion video. Unfortunately, there's no standard way to measure or specify response time, so different LCD makers use different methods. That means you can't necessarily compare the response time specified by one LCD maker directly with that specified by another. (Actually, it's worse than that; you can't necessarily compare response times for two different models made by the same company.)

When LCDs first appeared, most makers specified rise-and-fall response in milliseconds (ms), the time required for a pixel to change from black to white (rise time) and then from white to black (fall time), also called the black-white-black (bwb) response. Nowadays, in addition to or instead of bwb, many LCD makers specify white-black-white (wbw) response and/or gray-to-gray (gtg) response, the time required to go from one level of gray to another.

And gtg times are not necessarily comparable between different brands, or even between different models from the same company, because gtg time depends on which particular levels of gray are tested. Do we specify gtg response for going from an almost-black gray to an almost-white gray, or for going from one almost-middle gray to another almost-middle gray? It makes a difference.

Some makers also specify the rise time separately. For example, we found one display that was advertised as having a 4 ms response time, but the product data sheet on the maker's web site listed that display as having an 8 ms response time. Both numbers were accurate, as far as they went. The 4 ms time quoted in the ad referred to rise time (black to white). The 8 ms time quoted in the technical documents referred to bwb response.

It is not safe to make assumptions about one type of response time based on another type. For example, one LCD may have response times of 20 ms bwb and 8 ms gtg, while another model from the same manufacturer may have response times of 16 ms bwb and 12 ms gtg. So, is the second LCD slower or faster than the first? It depends on which numbers you decide to use. Advertisers use the fastest numbers available. Count on it.

All of these response-time numbers can be different, and there's no direct relationship among them. If you look only at ads (as opposed to technical documentation), it's often not clear what type of response time is being specified. If a response time is quoted without qualification, such as "16 ms," that ordinarily (but not always) refers to bwb response.

A fast bwb (or wbw) response time is more important for general use, while a fast gtg response time is more important for gamers and graphic artists. For general use, bwb response of 25 ms to 30 ms is acceptable to most people, and 16 ms to 20 ms preferable. For gaming and other demanding applications, bwb response of 12 ms is generally acceptable and 8 ms preferable, with gtg response no slower than 8 ms and 4 ms or less desirable.

LCD advantages

Relative to CRT displays, LCD displays have the following advantages:

Brightness

LCDs are brighter than CRTs. A typical CRT has brightness of about 100 candelas/square meter, a unit of measurement called a nit. (Some displays are rated in foot Lamberts (fL); one fL equals about 3.43 nits). A typical LCD is rated at 250 to 350 nits, roughly three times as bright as a typical CRT. CRTs dim as they age, although a brightness control with enough range at the upper end can often be used to set an old CRT to near original brightness. The CCRTs used to backlight LCDs also dim as they age, but generally fail completely before reduced brightness becomes a major issue.

Contrast

Contrast measures the difference in luminance between the brightest and dimmest portions of an image, and is expressed as a ratio. The ability to display a high-contrast image is an important aspect of image quality, particularly for text. An average CRT may have a contrast ratio of 200:1, and a superb CRT 250:1. An inexpensive LCD may have a contrast ratio of 400:1, and a superb LCD 1,000:1. In other words, even an inexpensive LCD may have higher contrast than an excellent CRT.

Usability in bright environments

Even good flat-screen CRTs are subject to objectionable reflections when used in bright environments, such as having the screen facing a window. Good LCDs are much superior in this respect. Short of direct sunlight impinging on the screen, a good LCD provides excellent images under any lighting conditions.

Size and weight

A typical CRT is about as deep as its nominal screen size. For example, a 19" CRT may be 19" from front to back. Large CRTs may be difficult to fit physically in the available space. Conversely, LCDs are quite shallow. The panel itself typically ranges from 1.5" to 3" deep, and even with the base most LCDs are no more than 7" to 8" deep. Also, where a large CRT may weigh 50 to 100 pounds or more, even large LCDs are quite light. A typical 17" LCD might weigh 10 pounds, and even a 23" unit may weigh less than 20 pounds. That small size and weight means that it's possible to desk- or wall-mount an LCD with relatively inexpensive mounting hardware, compared to the large, heavy, expensive mounting hardware needed for CRTs.

NOMINAL VERSUS ACTUAL SIZE

Stated LCD display sizes are accurate. For example, a 19" LCD has a display area that actually measures 19" diagonally. CRT sizes, on the other hand, are nominal because they specify the diagonal measurement of the entire CRT, part of which is covered by the bezel. For example, a nominal 19" CRT might have a display area that actually measures 18.1" diagonally. A couple of lawsuits several years ago convinced CRT makers to begin stating the usable size of their CRTs. This is stated as VIS (viewable image size or visible image size), and is invariably an inch or so smaller than the nominal size.

This VIS issue has given rise to the belief that a 15" LCD is equivalent to a 17" CRT, a 17" LCD to a 19" CRT, and so on. In fact, that's not true. The image size of a typical 17" CRT is an inch or so larger than that of a 15" LCD, as is the image size of a 19" CRT relative to a 17" LCD.

Power consumption

Depending on size and other factors, a typical CRT consumes 100 to 160 watts while operating, while an LCD consumes only a quarter to a half as much power. Using an LCD reduces your electricity bill directly by consuming less power and indirectly by reducing the heating load on your air conditioning during hot weather.

Choosing an LCD display

If you've decided that an LCD is right for you, use the following guidelines to choose one:

• Current LCDs are available in analog-only, digital-only, and models with both analog and digital inputs. Analog input is acceptable for 15" (1024x768) models, but for 17" (1280x1024) models analog video noise becomes an issue. At that screen size and resolution, analog noise isn't immediately obvious to most people, but if you use the display for long periods the difference between using a display with a clean digital signal and one with a noisy analog signal will affect you on almost a subconscious level. For a 19" (1280x1024) LCD, we regard a digital signal as extremely desirable but not absolutely essential. For a larger display or above 1280x1024, we wouldn't consider using analog signaling.
• Insist on true 24-bit color support, which may be described as support for 16.7 million colors. Most current LCDs support 24-bit color, allocating one full byte to each of the three primary colors, which allows 256 shades of each color and a total of 16.7 million colors to be displayed. Many early LCDs and some inexpensive current models support only six bits per color, for a total of 18-bit color. These models use extrapolation to simulate full 24-bit color support, which results in poor color quality. If an LCD is advertised as "24-bit compatible," that's good reason to look elsewhere. Oddly, many LCDs that do support true 24-bit color don't bother to mention it in their spec sheets, while many that support only 18-bit color trumpet the fact that they are "24-bit compatible."
• Most LCD makers produce three or more series of LCDs. Entry-level models are often analog-only, even in 19" and 21" sizes, and have slow response times. Midrange models usually accept analog or digital inputs, and generally have response times fast enough for anything except 3D gaming and similarly demanding uses. The best models may be analog/digital hybrids or digital-only, and have very fast response times. Choose an entry-level model only if you are certain that you will never use the display for anything more than word processing, web browsing, and similarly undemanding tasks. If you need a true CRT-replacement display, choose a midrange or higher model with a digital interface and the fastest response time you are willing to pay for.
• Decide what panel size and resolution is right for you. Keep in mind that when you choose a specific LCD model, you are also effectively choosing the resolution that you will always use on that display.
• Buy the LCD locally if possible. Whether or not you buy locally, insist on a no-questions-asked return policy. LCDs are more variable than CRT monitors, both in terms of unit-to-unit variation and in terms of usability with a particular graphics adapter. This is particularly important if you are using an analog interface. Some analog LCDs simply don't play nice with some analog graphics adapters. Also, LCDs vary from unit to unit in how many defective pixels they have and where those are located. You might prefer a unit with five defective pixels near the edges and corners rather than a unit with only one or two defective pixels located near the center of the screen.
• If you buy locally, ask the store to endorse the manufacturer's warranty that is, to agree that if the LCD fails you can bring it back to the store for a replacement rather than dealing with the hassles of returning the LCD to the maker.
• If possible, test the exact LCD you plan to buy (not a floor sample) before you buy it. Ideally, and particularly if you will use the analog interface, you should test the LCD with your own system, or at least with a system that has a graphics adapter identical to the one you plan to use. We'd go to some extremes to do this, including carrying our desktop system down to the local store. But if that isn't possible for some reason, still insist on seeing the actual LCD you plan to buy running. That way, you can at least determine if there are defective pixels in locations that bother you. Also, use a neutral gray screen with no image to verify that the backlight evenly illuminates the entire screen. Some variation is unavoidable, but one or more corners should not be especially darker than the rest of the display, nor should there be any obvious "hot" spots.

Recommended Brands: Our opinion, confirmed by our readers and colleagues, is that NEC-Mitsubishi, Samsung, Sony, and ViewSonic make the best LCDs available. Their LCDs particularly their midrange and better models provide excellent image quality and are quite reliable. You're likely to be happy with an LCD from any of these manufacturers.

• Stick with good name brands and buy a midrange or higher model from within that name brand. That doesn't guarantee that you'll get a good LCD, but it does greatly increase your chances. The LCD market is extremely competitive. If two similar models differ greatly in price, the cheaper one likely has significantly worse specs. If the specs appear similar, the maker of the cheaper model has cut corners somewhere, whether in component quality, construction quality, or warranty policies.

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