CRT Guide – A THG Primer – THG.RU

Introduction

The most expensive part of any computer system is almost always the monitor, yet most people don’t understand how their monitors work or how to shop for one. They tend to rely on a friend’s advice, past experience, reviews, a salesperson’s recommendations, price, or they just settle for the monitor bundled with their system. I’ve seen people invest hundreds of dollars on a state-of-the-art graphics board, and then connect a cheap and cheesy monitor to their system. Perhaps even more shameful are people who already own a good monitor, but don’t bother to keep it properly calibrated (or have never calibrated it at all). In this article, I’ll talk about the various kinds of monitors that are available and focus primarily on CRT (Cathode Ray Tube) monitors. I’ll give some tips on shopping for a new monitor and how to keep it properly calibrated.

We’ve already covered LCD displays in the following articles so, I thought it was worth giving the good ol’ CRT a run this time around:

• TFT Guide – Part 1 – Flat Panel Displays
• TFT Guide – Part 2 – Viewing Angle Technologies
• TFT Guide Part 3 – Digital Interfaces

CRT Basics

CRT displays, in the form of TV sets, have been around for about 60 years and, for the most part, their inner workings haven’t changed much. The basic idea is that an electron gun at the back of the picture tube fires a beam of electrons toward the inside front of the tube, which is coated with a layer of phosphorus material. The electron beam passes through a series of strong magnets that bend its path to strike different parts of the front of the tube. When the electron beam reaches the front glass, it strikes the phosphorous coating on the inside surface, causing that spot to glow temporarily. Each spot represents a single pixel (picture element). By carefully controlling the voltage of the electron beam, the individual spots can be made to glow brighter or dimmer. Originally, black and white TV picture tubes had one electron gun and a uniform coating of phosphors. Later, multiple guns were used and the phosphors were painted in discrete dots.

To create an image, the beam sweeps across a single horizontal line (scan line) from left to right, lighting up phosphor dots and causing them to glow brighter or dimmer, depending on the voltage. The speed that a monitor draws a single scan line, called the “horizontal frequency,” is measured in kilohertz (kHz). When the beam reaches the end of the line, it is turned off momentarily (called the “horizontal blanking interval”), the magnets reset, and the next line down is then painted. The process repeats, painting line after line, until the screen is filled. At that point, the beam is turned off again (called the “vertical blanking interval”), the magnets reset, and the whole process starts again at the top left of the screen. The speed that a monitor draws an entire screen, called the “vertical refresh rate” or “frequency,” is measured in hertz (Hz).

In the early days of television, the engineers who designed and built TV picture tubes faced technical problems that caused them to make some compromises. First, the quality of the phosphors available in the early days was not that great, and the dots would start to fade out before the entire screen could be painted. To get around this, televisions use an interlacing technique where the screen is painted in two passes, every other line at a time. On the first pass, only the odd scan lines (1, 3, 5, …) are painted. Then, the beam resets to the top, and the even scan lines (2, 4, 6, …) are painted. Each pass is called a “field,” and two fields combined are called a frame. In NTSC there are 60 fields (30 frames) painted per second, and in PAL TV systems there are 50 fields (25 frames) painted every second (movie film runs at 24fps, by the way). Any slower than that, and most people will begin to notice image flickering.

By the time computers came along, both the quality of the phosphors and that of the electronics had improved to the point where interlacing wasn’t necessary, but since most computer work involves lots of text, resolutions had to be increased. Where a typical TV set runs at about 13.5kHz horizontal refresh at a 25 to 30Hz vertical refresh rate, most computer monitors are capable of drawing a screen at over 60kHz horizontal refresh at an 85Hz vertical refresh rate. While no two people are alike, it is probably best to try and run a CRT monitor at 85Hz or better, in order to reduce eyestrain caused by flicker (even if you can’t actually see the flicker at lower frequencies).

Black And White To Color

A color CRT works in almost exactly the same way as a black and white CRT, except that instead of a single electron gun there are three, and instead of single-color phosphor dots there are triads of three different-colored phosphors (red, green, and blue) that make up each pixel. Each gun strikes a colored phosphor, and by adjusting the intensity of these three different-colored dots, it’s possible to create almost any color. (Actually, our brains do the color mixing.)

Adding color triads to a picture tube while maintaining high resolutions means crowding the phosphors closer and closer together. This requires even greater precision on the part of the electronics controlling the guns and the magnets. If the beams aren’t aimed precisely enough, they might strike adjacent phosphors (causing the picture to smear), produce incorrect colors, or create slight ghosting (causing the image to appear slightly out of focus).

To get around this problem, engineers use a couple of tricks. One solution is to use a mask (called a “shadow mask”) placed inside the tube, just in front of the phosphor-coated surface. The mask is simply a sheet of metal (usually made of a material called “invar”), with holes drilled in it. Only beams that are correctly aimed will pass through the holes that correspond to the particular phosphor dots. The mask blocks any stray beams from striking the wrong phosphors. This is the shadow mask approach.

Painting phosphor dots on the inside of a tube can be a tricky business, so another manufacturing trick was used. Instead of dots, the phosphors are painted in vertical lines and very thin wires are run in front, rather than a mask. The thin wires serve to block stray electron beams in the same way as a shadow mask. This is the “aperture grille” approach.

Both of these techniques have their trade-offs. In general, shadow mask CRTs produce a sharper image, while aperture grille CRTs produce better color. Shadow mask CRTs tend to be a little dimmer, while aperture grille CRTs have two horizontal damping wire shadows (to reduce grille vibrations) at 1/3 and 2/3 across the screen. If you want bright, accurate colors (and don’t mind the two very faint damping wire shadows), go for an aperture grille CRT. If you do a lot of text work or find the shadows annoying, then go with a shadow mask CRT. If desk space or absolutely perfect geometric reproduction is required, then you might want to consider an LCD instead.

Dot Pitch Mythology

In trying to evaluate a monitor’s quality, most people will usually talk about dot pitch. In general, the lower the dot pitch (measured in millimeters), the better the monitor. The problem is that dot pitch can be measured in many different ways, and therefore doesn’t necessarily mean much. Traditionally, a shadow mask CRT’s dot pitch is the distance between two of the same-colored phosphor dots (measured diagonally from one scan line to the next). However, in an aperture grill CRT there are no dots (only stripes), so dot pitch (or more accurately, stripe pitch) – is measured horizontally, between two of the same-colored stripes. For marketing purposes, shadow mask manufacturers started quoting horizontal dot pitch, too. There are also a few companies that publish their mask pitch instead. However, since the mask is about 1/2″ behind the phosphor surface of the screen, a .21mm mask pitch might actually translate into a .22mm phosphor dot pitch by the time the beam strikes the screen. Finally, because CRT tubes are nearly (but not completely) flat, the electron beam tends to spread out into an oval shape as it reaches the edges of the tube, so some manufacturers will spread the dot pitch wider toward the edges. Some manufacturers will publish two dot pitch measurements, one for the center of the screen, and the second for the outermost edges.

While the number and tightness of triads (or stripes) determines the optimum resolution, a monitor’s ability to accurately strike precise phosphors (called “convergence”) is even more important when it comes to creating a sharp image. If the convergence isn’t quite right, an electron beam aimed at a blue phosphor (or stripe) might strike part of the red or green phosphor just next to it. Horizontal or vertical convergence errors (measured in tenths of millimeters) can cause color ghosting and make text harder to read or fine details appear fuzzy. Unfortunately, only a few monitor manufacturers publish their convergence specifications.

Matching Monitors To Graphics Cards

Monitors (and TV sets) are essentially analog devices. No matter how much you try to manipulate the tube components, they are still analog. However, computers operate in a digital world, so at some point (usually within the graphics board) the digital images created in the computer must be converted to analog signals in order for the monitor to display anything.

The key component in a graphics card that determines how well (or if) it can drive a monitor is the speed of its RAMDAC (Random Access Memory Digital Analog Converter), measured in megahertz. The RAMDAC is usually a separate component on the graphics board, though sometimes it is incorporated in the graphics chip, and sometimes (as in the case of DVI, digital video interface), it is in the monitor.

The following table shows minimum RAMDAC requirements for different resolutions running at different refresh rates.

Shopping Around

In an ideal world, you could go into a store and see dozens of monitors, and they’d already be connected to PCs with the same graphics board that you will be using at home, and under the same lighting conditions. You would install monitor calibration software (such as that from DisplayMate Technologies) on each of the systems, spend a few hours calibrating each monitor and run through the diagnostics. When you finally select the best monitor for your needs, the friendly employee would disconnect it, box it back up in the original carton, and you’d be able to take that particular monitor home with you. Unfortunately, it is not an ideal world, and there aren’t many stores that would allow you to do that. Even if you could test a few monitors on the show floor, they would probably give you one from their storeroom of- the same make and model, but not the exact monitor you had seen on the floor.

That brings up an important point – no two monitors are exactly alike. I’m not just talking about differences between brands, or even model numbers. Even units of the same model from the same manufacturer can perform quite differently. Sometimes you get lucky – sometimes not.

If you’re in the market for a new monitor, there are a few things to consider before you start. Perhaps the most important factor is matching the monitor to your normal operating screen resolution. In the old days, monitors were fixed at certain frequencies and resolutions (LCD panels still are), but these days most CRT monitors will run at a variety of frequencies and resolutions. But even though a multi-scan monitor will function at different frequencies and resolutions, some CRT monitors function better at certain resolutions and strain in order to run at others, which shortens their life. A more expensive monitor designed to run best at 1600 x 1200 won’t look quite as good if you run it at 1024 x 768. Unfortunately, many monitor manufacturers tend to highlight their maximums and downplay their optimum settings. In general, most monitors are designed to operate at peak efficiency at about 85Hz. If a monitor data sheet gives you a wide range of possible resolutions at different frequencies, look at the 85Hz values to get an idea of what its optimal settings should be.

• 85Khz Class = 1024 x 768 @ 85Hz
• 95Khz Class = 1280 x 1024 @ 85Hz
• 107Khz Class = 1600 x 1200 @ 85Hz
• 115Khz Class = 1600 x 1200 @ 92Hz
• 125Khz Class = 1856 x 1392 @ 85Hz

If you normally run at 1600 x 1200 resolutions, then you should look for a monitor in the 107 or 115 Khz class. Using a lower class monitor will require overdriving it, which will not only give a generally lower-quality image; but may also reduce the life of the monitor. On the other hand, if you normally run at 1024 x 768 or lower, then buying a higher-class monitor and running it at lower resolutions can produce moirй patterns and give you less than optimal performance (not to mention the fact that you are also wasting money).

If you want to get the best picture from an existing monitor, you can work the numbers backwards. Take the horizontal size of the display area and divide by the dot pitch; then, set the resolution to the nearest setting just below that. For example, a 19″ monitor typically has a horizontal viewable area of 360mm. Let’s assume it has a 0.22mm horizontal dot pitch. 360 / 0.22 = 1636 dots across the screen. So, the best setting is probably 1600 x 1200 at 85Hz. A monitor with the same viewable area and a 0.24mm horizontal dot / aperture pitch has 1500 dots. So, the best setting would probably be 1280 x 1024.

Vertical resolution isn’t as problematic. A typical 19″ monitor has a vertical viewable area of 270mm. Because aperture grill monitors use vertical stripes, they have a 0.00mm vertical aperture pitch, so vertical resolution capability is virtually unlimited. As with every technology, there’s a trade-off – here, the downside is that more beam current hitting the phosphors makes them more susceptible to screen burn. Use a screen saver!

Most shadow mask monitors have a 0.14mm vertical dot pitch. 270 / 0.14 = 1928 lines – far more than any current video card can produce at any decent refresh rate.

The pixel rate and the horizontal scan frequency determine the resolution and refresh rate of the monitor.

Setting Up A Monitor

In theory, all monitors are set to their optimum settings in the factory, so you should never have to adjust anything. If you’re shopping for a new monitor and find that you have to change the settings much in order to get a good image, then you should probably look for a different monitor. However, there are numerous factors that can cause a monitor to need some adjusting, now and then.

Although the human eye is a very sensitive light-measuring instrument, just looking at a monitor won’t tell you much about its quality. Unless you know exactly what to look for, you’re going to need a little help when evaluating a monitor. I would strongly recommend getting a display calibration package such as those from DisplayMate Technologies. Their software can help you calibrate and adjust any monitor, as well as help you identify any problem areas. Their test screens are specially designed to accentuate specific display properties one at a time, making it easier to see things like geometric distortions, color accuracy, convergence problems, focus, moirй patterns, glare, etc. Trying to detect these kinds of problems without good calibration screens is very difficult, though not impossible.

Most people don’t calibrate their monitors ever, let alone on a regular basis. It is worth the time and effort, though, particularly if you are working in video or print. With the proper software, it isn’t too difficult to get your monitor in the best possible shape, and once you become familiar with what to look for, keeping it properly calibrated should only take a few minutes every month or so. You can also use these calibration techniques to evaluate monitors you’re thinking about buying. Even without the proper test screens, you should be able to do a rough calibration in any store (if you know what you’re looking for) and get a good idea of the monitor’s performance.

When calibrating any monitor, start by adjusting everything to the factory settings (including the graphics card settings). Set the resolution you’ll be using most of the time, using the graphics card settings panel. As I mentioned earlier, if possible, try to run your monitor at 80 to 85Hz or higher. This will reduce eyestrain. Wait a while before you begin calibrating the monitor. CRT monitors need time to warm up and settle before you start calibrating. I usually let a monitor warm up for at least 20 minutes before starting, and even then I run through the brightness and contrast calibrations a few times in between the other tests, just to make sure that it hasn’t ‘drifted’ since I began. Of course, if you’ve got a good monitor, you shouldn’t have to do much waiting or calibrating.

When adjusting any monitor, the first calibration step is setting the brightness and contrast levels properly. Oddly enough, brightness controls actually adjust the black levels of the CRT, while contrast controls adjust the maximum white levels. Higher contrast will make things ‘pop’ on the screen and can make them seem sharper and brighter, while lower contrast gives you more shades ,at a cost of ‘smoothing’ the image and losing a bit of brightness. Ultimately, you want a contrast balance where you can see the maximum number of shades of gray, yet whites look nearly white and blacks are nearly black.

Most people tend to run their monitors (and TV sets) a little too ‘hot,’ since brighter seems to look better (particularly in an office environment, with lots of lights and windows). If your brightness and contrast settings are at their maximum, you won’t get the best picture and you can shorten the life of the monitor (monitors should last between three and five years).

Setting Up A Monitor, Continued

The first thing to do when setting up (or evaluating) a monitor is adjust the viewable area so that it almost fills the screen. If you have to stretch things a lot to nearly fill the screen, then that can be a sign that the manufacturer is trying to hide edge-conversion or geometric distortion problems. Some monitor manufacturers will sometimes cheat a bit and reduce the viewable screen size, in order to hide problems at the edges. While this can reduce the severity of the problems, you can end up being cheated when it comes to viewable screen area. Under normal conditions, a 17″ CRT should give you at least a 16″ (measured diagonally) viewing area. If a monitor defaults to anything lower than that, you might be paying for a 17″ monitor while only getting a 15″. Keep in mind that if you expand an image horizontally or vertically by different amounts, you’ll distort the images (circles won’t be round and squares won’t be square).

The next step is to adjust the brightness. The easiest way to do this is to pull up a black screen, if you have one, then lower the brightness until the display area is almost (but not quite) as black as the surrounding border. You’ll probably feel that this makes the image a little on the dim side, as the whites will be a bit more gray than usual. Next, you need to adjust the contrast. White text on black is a good way to test for this. Turn the contrast all the way up and then start lowering it until the whites are nearly white, without being blurry (or until it feels good on the eyes).

Adjusting the size, brightness and contrast properly will probably make your display seem too dark, but after a while you’ll find that the images are sharper and colors more accurate.

At this point you are 95% finished, but there are some more advanced adjustments you can make. With a crosshatch pattern screen (like that in DisplayMate’s Video Torture Test series), you can fine-tune the geometric aspects of the display.

First, adjust the horizontal and vertical size controls to try and get the crosshatch to form as close to a perfect square as possible. Once that is done, go back and adjust the size to nearly fill the screen, and then center the image. Next, if necessary, adjust the rotation (if your monitor allows this) so that the image isn’t tilted.

Next, you’ll need to adjust the horizontal pincushion controls to eliminate any ‘hourglass’ or horizontal ‘bulge’ shape. The edges on the sides of the screen should be as close to perfectly straight as possible. This can be a tricky adjustment, as your monitor may have one side in and the other out, or the left side might be straight while the right side is bowed in or out, or the top or the bottom third of the screen might be curved in or out.

Depending on the monitor controls, you may be able to adjust for any or all of these problems, but again, if you have to make a lot of adjustments, you might consider buying a different monitor.

Color Temperatures

If you’ve played around with your monitor’s on-screen display controls (OSD), you probably ran into a color temperature setting. Most monitors will let you adjust the color temperature (measured in kelvin) to one of three settings or more. The standard default settings are usually 9300 kelvin, 6500 kelvin, and 5500 or 5000 kelvin. 9300 kelvin is usually the default setting, and is sometimes referred to as “computer monitor white.” This setting will give you the brightest picture, but is slightly bluish in color. 6500 kelvin provides a whiter white, sometimes referred to as “daylight white.” People in the video world prefer this setting. The 5500 or 5000 kelvin setting is sometimes referred to as “paper white,” and is commonly used in the print and prepress world. If you’re doing a lot of desktop publishing or color printing then you might want to use this setting.

While some monitors will allow you to set your own temperatures or even play with the individual color values, you should probably avoid changing things too much, unless you have a specific purpose in mind and know what you are doing. For general computer use, you probably want to stick with the default 9300 kelvin setting.

Convergence Corrections

You should never have to adjust the convergence settings with a good monitor, but it’s nice to know how, should the situation arise. As I mentioned earlier, convergence is probably the most important factor in producing the sharpest picture. Professionals use a convergence gauge to measure convergence errors, but DisplayMate has special screens that will show convergence errors.

You can also use the business card trick (showed to me by Jim Witkowski at Monitors Direct/Cornerstone). Put up a dark background with a thin, horizontal white line. Now, hold a business card up to the screen and slowly move it up, so that it crosses the line at a slight angle. If the monitor’s vertical convergence is off, you’ll see a red, green or blue fringe on the edge of the card. This trick also works for detecting horizontal convergence errors.

Buying Tips

If at all possible, buy your monitor from a retail store, rather than mail order over the Web. It might cost you an extra $25, but consider this: many mail order houses will charge you a restocking fee or won’t pay for shipping if you have to return a monitor. Shipping a 90-pound monitor will cost you far more than the $25 you “saved.”

If you purchase your monitor at a store make sure that there won’t be any additional fees or charges if you have to exchange the monitor. Also, make sure that you don’t have to try and have the monitor repaired before they’ll exchange it. You could be waiting for weeks, only to get the same monitor back (with the same problems), and have to go through the whole process again.

Make sure you can exchange the monitor for any reason. Problems can, and do, appear, even if the monitor is ‘working properly.’ In other words, there are problems that a retailer might not consider problems, as long as the monitor shows any sort of picture. For example, some monitors have focus problems (particularly in corners). You might be able to adjust the monitor to alleviate the focus problem in three corners, but never be able to fix all four corners. Will your dealer accept this as a good enough reason to exchange the monitor for a new one?

If you are shopping in a store, set the screen to the same resolution that you normally use. Reset the monitor to the factory settings. Measure the viewable area – if the borders are extra wide, it may be hiding other problems. Look for any obvious distortion problems – a maximized window should have nice, straight edges on the sides, top and bottom, and particularly in the corners. Adjust the brightness and contrast as described above. Then bring up a white or solid color screen, if possible (you can set the background image to none in the display properties window or even bring up a blank text screen). Look for any variations in brightness across the entire screen. Monitor brightness can vary as much as 30% from center to corners, but this can cause eyestrain. If you see circular shadows or uneven brightness, move on to another model. Ideally, your monitor should have even brightness over the entire screen. You can then do the business card test to give you an idea about convergence. If you detect convergence problems, don’t bother to try and adjust the monitor in the store, just move on to another. Finally, check the specifications. Make sure the monitor falls into the proper Class range for your preferred resolution.

Conclusions

There are many factors that influence the quality of a monitor – convergence, dot pitch, matching the graphics card to the monitor, running the monitor at its optimum resolution, and making sure that it is properly calibrated. There are other factors that are harder to judge without opening the monitor’s case, such as how much shielding is used, how many circuit boards are used, how the wires are secured, power supply, etc. (NOTE: DO NOT OPEN YOUR MONITOR! There are high voltages inside a monitor that can kill you, even if it is unplugged!)

There are also a lot of factors that aren’t very important. The number and layout of the OSD (on-screen display) controls don’t really matter, as you shouldn’t have to adjust things very often. The connector type isn’t that important, either. In general, an integrated cable (one that doesn’t unplug from the monitor) has the advantage of soldered[S1] connections that usually provide the best connection. BNC cables tend to be a little more robust than 15-pin Mini D Sub cables, but, because there are five separate connections, they can be a little tricky to set up. Also, they don’t support Window’s Plug and Play. There is virtually no visual quality difference between BNC and Mini D Sub connections. DVI connections are less prone to degradation and interference as the signals travel across the cable (not really a problem with good cables), but the DVI standard has not yet been extended to high-resolution monitors that have high refresh rates. DVI is mostly used for LCD displays that run at relatively slow refresh rates.

Multiple connectors, headphone jacks, built-in speakers, and the like can be nice, but they are mostly just frills, and have nothing to do with the quality of the image. Special glare reduction coatings are nice if you have a lot of light in your work environment, but almost all monitor manufacturers use the exact same coatings – the only difference is in the amount. Thicker coatings eliminate more glare, but reduce the amount of light emitted. If you have glare problems, it’s probably better to try and change your lighting environment, rather than choosing your monitor based on glare reduction coatings. Power consumption also shouldn’t be much of an issue as most CRTs only consume about 130 watts (LCDs consume about half that, but the higher initial cost of an LCD pretty much offsets any savings, in the long run).

Depending on your situation and the types of applications you run most often, different factors might be more or less important. If you do a lot of text work, note that shadow mask CRTs tend to give a sharper picture. If accurate color reproduction is more important to you, then an aperture grille CRT might be a better choice. If desk space is at a premium, then LCDs might be a good alternative though they are fixed resolution, and generally not well suited to video or animation. Flat-screen CRTs and LCDs offer the best glare reduction, but tend to be more expensive and heavier. Flat-screen CRTs also tend to suffer from geometric distortions at the edges and can appear to give a slightly concave image. Because of their design, LCDs have no geometric distortion problems, making them ideal for CAD work.

While it’s possible to get an unusually good, cheap, off-brand monitor (or a lemon from a top-notch manufacturer), in general, you can expect to get what you pay for. Do a little homework, check the specs, read reviews, match the monitor to your system and preferences, and make sure you can exchange it if you happen to get a dud. You’re going to be staring at your monitor for a long, long time, so it’s worth getting the best you can afford.