配件 & 工具
Learn what a CRT display is, and how CRTs work.
== CRT Displays == CRT displays use the same picture tube technology with many enhancements as the first color televisions did more than half a century ago. But old doesn't necessarily mean obsolete. A good CRT display, such as the Samsung 997DF 19" model shown in '''Figure 11-1''', provides excellent image quality at a reasonable price. CRT displays are an excellent choice for many people, and will remain so for years. [image|232166|size=medium|align=center|caption=Figure 11-1: Samsung 997DF 19" CRT display (image courtesy of Samsung)] [quote|format=featured]'''OLD FRIENDS AND NEW ONES''' In autumn 2005, Robert finally replaced his beloved Hitachi SuperScan Elite 751 19" CRT display which he had been using as his primary display for six years with a 19" Samsung 930BF LCD display. The Hitachi is a top-notch display, and Robert would have sworn that its image quality was as good then as the day it was first installed. Until, that is, he connected the Samsung 930BF. The difference was startling. The Samsung provided much better brightness, contrast, and color saturation. Does that mean that a good LCD display always beats a good CRT display, or that current display technology is worlds better than that of six years ago? Nope. It just means that every CRT display even the best models decreases in brightness, contrast, and saturation as it ages. From day to day, the difference is imperceptible, but as the months and years pass the accumulated difference becomes large. There is a happy ending to this story, though. Robert had been running the Hitachi CRT at 50% brightness and 85% contrast for years. Boosting brightness to 75% and contrast to 100% greatly improved the display quality, so there's life in it yet. Barbara promptly grabbed the Hitachi for her own office, where it will probably live for another few years.[/quote] CRT monitors use the following major components: ==== CRT ==== The CRT is essentially a large glass bottle, flat or nearly so on one end (the screen), tapering to a thin neck at the back, and with nearly all air exhausted. The inside of the screen end is covered with a matrix of millions of tiny ''phosphor'' dots (or stripes). A phosphor is a chemical compound that, when struck by electrons, emits visible light of a particular color. Phosphors are organized by groups of three, collectively called a ''pixel''. Each pixel contains one phosphor dot that emits each of the additive primary colors, red, green, and blue. By choosing which dots to illuminate and how brightly to illuminate each, any pixel can be made to emit any one of thousands or millions of discrete colors. The distance between nearest neighbors of the same phosphor color on adjacent rows is called the ''dot pitch'' or ''stripe pitch''. A smaller pitch results in a sharper image and the ability to resolve finer detail. ==== Electron guns ==== The phosphor dots are excited by one or more electron emitters, called ''electron guns'', located in the neck at the back of the monitor. A gun comprises a heated cathode, which emits electrons, and circuitry that focuses the free electrons into a thin beam. ==== Deflection yoke ==== The ''deflection yoke'' is located around the tapered portion of the CRT, between the guns and the screen. This yoke is actually a large electromagnet, which, under the control of the monitor circuitry, is used to steer the electron beam(s) to impinge on the correct phosphor dot at the correct time and with the correct intensity. ==== Mask ==== The ''mask'' sits between the electron guns and the phosphor layer, very close to the latter. This mask may be a sheet of metal with a matrix of fine perforations that correspond to the phosphor dot triads on the screen, called a ''shadow mask'', or a series of fine vertical wires that correspond to phosphors laid down in uninterrupted vertical stripes, called an ''aperture grill''. In practice, and despite the marketing efforts of manufacturers to convince us otherwise, we find that the mask type makes little real difference. Good (read: more expensive) monitors produce good images, regardless of their mask type. Inexpensive monitors produce inferior images, regardless of their mask type. === CRT Characteristics === Here are the important characteristics of CRT monitors: ==== Screen size ==== ''Screen size'' is specified in two ways. The nominal size the size by which monitors are advertised and referred to is the diagonal measurement of the tube itself. However, the front bezel of the monitor conceals part of the tube, making the usable size of the monitor less than stated. Various consumer lawsuits have resulted in monitor manufacturers also specifying the ''Viewable Image Size (VIS)'', which is the portion of the tube that is actually visible. Typically, VIS is an inch or so less than nominal. For example, a nominal 17" monitor may have a 15.8" VIS. Small differences in VIS for example, 15.8" versus 16" make little practical difference. The smallest monitors still available are 15". While 17" remains the most popular size, 19" models are now so inexpensive that they have nearly overtaken 17" models in unit sales. Monitors 21" and larger are still relatively expensive, and are used primarily by graphics artists and others who require huge displays. ==== Dot/stripe pitch ==== ''Dot pitch'' or ''stripe pitch'' is measured in millimeters, and specifies the center-to-center distance between the nearest neighboring phosphor dots or stripes of the same color. Smaller pitch means a sharper image that resolves finer detail. Unfortunately, dot pitch, which is used to describe shadow mask monitors, cannot be compared directly to stripe pitch, which is used to describe aperture grill monitors. For equivalent resolution, stripe pitch must be about 90% of dot pitch. That is, a 0.28 mm dot pitch monitor has resolution similar to a 0.25 mm stripe pitch monitor. ==== Maximum resolution ==== ''Maximum resolution'' specifies the maximum number of pixels that the monitor can display, which is determined by the physical number of pixels present on the face of the tube. The maximum resolution of many low-end monitors is identical to the optimum resolution for that monitor size. For example, 1024x768 is optimum for 17" monitors, so many low-end 17" monitors provide 1024x768 maximum resolution. Conversely, midrange and high-end monitors may have maximum resolutions higher than practically usable. For example, a high-end 17" monitor may support up to 1600x1200. There is no real benefit to such extreme resolutions, although it can be useful to have one step higher than optimum (e.g., 1280x1024 on a 17" monitor or 1600x1200 on a 19" monitor) available for occasional use for special purposes. ==== Synchronization range ==== The ''synchronization range'' specifies the bandwidth of the monitor, which determines which combinations of resolution, refresh rate, and color depth can be displayed. Synchronization range is specified as two values: ==== Vertical Scanning Frequency ==== ''Vertical Scanning Frequency (VSF)'' is the inverse of the time the monitor requires to display one full screen. VSF (also called ''refresh rate'') is measured in hertz (Hz) and specifies the number of times per second the screen can be redrawn. To avoid screen flicker, the monitor should support at least 70 Hz refresh at the selected resolution. Within reason, higher refresh rates provide a more stable image, but rates beyond 85 or 90 Hz are necessary only for specialized applications such as medical imaging. Most monitors support a wide range of refresh rates, from very low (e.g., 50 Hz) to very high (e.g., 120 to 160 Hz). ==== Horizontal Scanning Frequency ==== ''Horizontal Scanning Frequency (HSF)'' is the inverse of the time the monitor requires to display one full scan line. HSF is measured in kilohertz (KHz), and specifies the overall range of bandwidths supported by the monitor. For example, a monitor running 1280x1024 at 85 Hz must display 1024 lines 85 times per second, or 87,040 scan lines per second, or about 87 KHz. In fact, some overhead is involved, so the actual HSF for such a monitor might be 93.5 KHz. Resolution and refresh rate are interrelated parts of synchronization range of an analog monitor. For a given resolution, increasing the refresh rate increases the number of screens (and accordingly the amount of data) that must be transferred each second. Similarly, for a given refresh rate, increasing the resolution increases the amount of data that must be transferred for each screen. If you increase resolution or refresh rate, you may have to decrease the other to stay within the HSF limit on total bandwidth. Note that manufacturers often specify maximum resolution and maximum refresh rate independently, without consideration for their interrelatedness. For example, specifications for a 19" monitor may promise 1600x1200 resolution and 160 Hz refresh. Don't assume that means you can run 1600x1200 at 160 Hz. 160 Hz refresh may be supported only at 640x480 resolution; at 1600x1200, the monitor may support only 70 Hz refresh. [quote|format=featured]'''COLOR DEPTH AND BANDWIDTH''' Resolution and refresh rate alone determine the required bandwidth for an analog monitor. Color depth is immaterial, because the color displayed for a given pixel is determined by the analog voltages present on the red, green, and blue lines at the time that pixel is processed. Therefore, at a given resolution and refresh rate, an analog monitor uses exactly the same bandwidth whether the color depth is set to 4, 8, 16, 24, or 32 bits, because the video card converts the digital color data to analog signals before sending it to the monitor. For purely digital monitors, such as LCD displays, greater color depth requires greater bandwidth, because color information is conveyed to a digital monitor as a digital signal.[/quote] ==== Tube geometry ==== Monitors use one of three geometries for the front viewing surface. Older monitors used spherical tubes or cylindrical tubes, both of which have noticeably curved surfaces. ''Flat square tubes (FST)'' are nearly flat. Other than some "value" models, all current monitors use an FST. Don't consider buying a monitor that is not FST. === CRT advantages === CRT displays have many advantages relative to LCD displays. ==== Price ==== CRTs cost less than LCDs. For the same price as an entry-level 17" LCD, you can buy a midrange 19" CRT or two good 17" CRTs. The pricing differential has somewhat narrowed recently, but LCDs are likely for the foreseeable future to cost more than CRTs with similar size, features, and quality. ==== Variable resolution ==== LCDs are designed to operate at one resolution, typically 1024x768 for 15" models and 1280x1024 for 17", 18", and 19" models. Although you can run an LCD at lower resolution than it was designed to use, you don't want to. At nonnative resolution, you must choose between having a sharp image that occupies only a portion of the screen or using pixel extrapolation, which results in a full-screen image with significantly degraded image quality. CRTs, conversely, can operate at various resolutions, which means that you can choose the resolution that suits your own preferences and vision. ==== Service life ==== A high-quality CRT normally lasts for many years. It's common for a CRT to remain in use for five years or more, and even ten years is not unheard of. LCDs use an array of ''cold cathode ray tubes (CCRTs)'', which are similar to fluorescent tubes, to provide the backlight required to view the image. A failed CCRT is not economically repairable. When a CCRT burns out, the LCD display must be replaced. [quote|format=featured]'''ONLY THE BAD DIE YOUNG''' Not all CRTs have a long service life. High-quality models typically live a long time; cheap models often die after only a year or two of use.[/quote] ==== Superior display of fast-motion video ==== CRTs use phosphor pixels, which can be turned on or off almost instantly. LCDs use transistorized pixels that respond more slowly. This slower response may be visible as a smearing or ghosting effect when an LCD displays fast-motion video, such as DVD video or graphics-intensive games. Although better LCDs don't exhibit this problem, at least not as severely as cheaper models, it is common and intrusive with entry-level LCDs. ==== Larger viewing angle ==== CRTs present essentially the same image quality regardless of viewing angle. Conversely, LCDs present their best image quality only within a relatively small viewing angle, although midrange and better LCD models typically have larger viewing angles than entry-level models. ==== Accurate color reproduction ==== Many graphic artists refuse to use LCDs because the appearance of colors and the relationship between them changes with viewing angle. This problem is particularly acute with inexpensive LCDs, although even premium units exhibit it at least to some extent. The best LCD models are good enough in this respect for routine use, but most who insist on accurate color reproduction still prefer high-quality CRT monitors. ==== No pixel defects ==== A CRT never has defective pixels. An LCD panel is manufactured as a monolithic item that contains more than a million pixels, and on some LCD panels one or a few of those pixels are defective. Defective pixels may be always-on (white), always-off (black), or some color. People vary in their reaction to defective pixels. Many don't even notice a defective pixel or two, while others, once they notice a defective pixel, seem to be drawn to that pixel to the exclusion of all else. Most manufacturer warranties specifically exclude some number of defective pixels, typically between five and ten, although the number may vary with display size and, sometimes, with the location of the defective pixels and how closely they are clustered. As long as the display meets those requirements, the manufacturer considers the display to be acceptable. You may or may not find it acceptable. ==== Vibrant color rendering ==== Although the contrast and brightness of recent high-end LCDs are excellent, most LCDs provide subjectively less vibrant color than a good CRT. This is particularly evident in the darkest and lightest areas, where tones seem to be compressed, which limits subtle gradations between light tones or dark tones that are readily evident on a good CRT. Also, some LCDs add a color cast to what should be neutral light or dark tones. For example, dark neutral tones may appear shifted toward the blue (cooler) or red (warmer) ranges. This problem is less prevalent in high-quality LCDs than in entry-level units, and is also more likely to occur if you are using an analog interface rather than a digital interface. If your budget is limited, a CRT offers far more bang for the buck than an LCD and, particularly for entry-level models, overall display quality will also be higher. === Choosing a CRT display === Use the following guidelines when choosing a CRT display: * Remember that a CRT display is a long-term purchase. Even with heavy use, a high-quality CRT can be expected to last five years or more, so buy quality and choose a model that's likely to keep you happy not just for your current system, but for one or even two systems after that. * Make sure the CRT is big enough, but not too big. We consider 17" models suitable only for casual use or those on the tightest of budgets. For not much more, you can buy a 19" model that you'll be much happier with. Conversely, make sure your desk or workstation furniture can accommodate the new CRT. Many people have excitedly carried home a new 21" CRT only to find that it literally won't fit where it needs to. Check physical dimensions and weight carefully before you buy. Large CRTs commonly weigh 50 lbs. or more, and some exceed 100 lbs. That said, if you find yourself debating 17" versus 19" or 19" versus 21", go with the larger model. But note that if your decision is between a cheap larger CRT and a high-quality smaller one for about the same price, you may well be happier with the smaller CRT. A $130 17" CRT beats a $130 19" CRT every time. * 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 CRT, but it does greatly increase your chances. The CRT 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. [quote|format=featured]'''RECOMMENDED BRANDS''' Our opinion, which is shared by many, is that NEC-Mitsubishi, Samsung, and ViewSonic make the best CRTs available. Their CRTs, particularly midrange and better models, provide excellent image quality and are quite reliable. You're likely to be happy with a CRT from any of these manufacturers.[/quote] * If possible, test the exact CRT you plan to buy (not a floor sample) before you buy it. Ask the local store to endorse the manufacturer's warranty that is, to agree that if the CRT fails you can bring it back to the store for a replacement rather than dealing with the hassles of returning it to the manufacturer. Mass merchandisers like Best Buy usually won't do this they try to sell you a service contract instead, which you shouldn't buy but small local computer stores may agree to endorse the manufacturer's warranty. If the CRT has hidden damage from rough handling during shipping, that damage will ordinarily be apparent within a month or two of use, if not immediately. [quote|format=featured]'''BUY CRTS LOCALLY''' After shipping costs, it may actually cost less to buy locally, but that is not the main reason for doing so. Buying locally gives you the opportunity to examine the exact CRT you are buying. CRTs vary more between samples than other computer components. Also, CRTs are sometimes damaged in shipping, often without any external evidence on the CRT itself or even the box. Damaged CRTs may arrive DOA, but more often they have been jolted severely enough to cause display problems and perhaps reduced service life, but not complete failure. Buying locally allows you to eliminate a "dud" before you buy it, rather than having to deal with shipping it back to the vendor or manufacturer.[/quote] * Most mainstream CRT manufacturers produce three Good, Better, and Best models in 17", 19", and 21". In general, the Good model from a first-tier maker corresponds roughly in features, specifications, and price to the Better or Best models from lower-tier makers. For casual use, choose a Good model from a first-tier maker, most of which are very good indeed. If you make heavier demands on your CRT such as sitting in front of it eight hours a day you may find that the Better model from a first-tier maker is the best choice. The Best models from first-tier makers are usually overkill, although they may be necessary if you use the CRT for CAD/CAM or other demanding tasks. Best models often have generally useless features like extremely high resolutions and unnecessarily high refresh rates at moderate resolutions. It's nice that a Best 17" model can display 1600x1200 resolution, for example, but unless you can float on thermals and dive on rabbits from a mile in the air, that resolution is likely to be unusable. Similarly, a 17" CRT that supports 115 MHz refresh rates at 1024x768 is nice, but in practical terms offers no real advantage over one that supports an 85 or 90 MHz refresh. * Choose the specific CRT you buy based on how it looks to you. Comparing specifications helps narrow the list of candidates, but nothing substitutes for actually looking at the image displayed by the CRT. For example, CRTs with Sony Trinitron tubes have one or two fine horizontal internal wires whose shadows appear on screen. Most people don't even notice the shadow, but some find it intolerable. * Make sure the CRT has sufficient reserve brightness. CRTs dim as they age, and one of the most common flaws in new CRTs, particularly those from second- and third-tier manufacturers, is inadequate brightness. A CRT that is barely bright enough when new may dim enough to become unusable after a year or two. A new CRT should provide a good image with the brightness set no higher than 50%. Like all other component manufacturers, CRT makers have come under increasing margin pressures. A few years ago, we felt safe in recommending any CRT from a first-tier maker, because those companies refused to put their names on anything but top-notch products. Alas, first-tier makers have been forced to make manufacturing cost reductions and other compromises to compete with cheap Pacific Rim CRTs. Accordingly, low-end models from first-tier makers may be of lower quality than they were in the past. The presence of a first-tier maker's name plate still means that CRT is likely to be of higher quality than a similar no-name CRT, but is no longer a guarantee of top quality. Many first-tier CRTs are actually made in the same Pacific Rim plants that also produce no-name junk, but don't read too much into that. First-tier CRTs are still differentiated by component quality and the level of quality control they undergo. There is no question in our minds that the first-tier CRTs are easily worth the 10% to 20% price premium they command relative to lesser brands. In fact, we think it is worth the extra cost to buy not just a first-tier CRT, but a midrange first-tier CRT. ''[[Displays|More about Displays]]''