Not many techs take the time to understand what goes on inside a monitor, what differentiates one monitor from another, and how to test a monitor for proper operation. In many companies, the belief is that it's easier to just replace a monitor than to find the cause of the problem. But that might not always be so. Knowing how a monitor works and where to look when problems arise can help you decide if it's worth your time and effort to fix the monitor or if you should just unbox a new one. Here are some enlightening terms and tips to help you out.
How CRT monitors work
The most common type of monitor is the cathode-ray tube, or CRT. It’s essentially a large vacuum tube. At the back of it is a long narrow neck containing a cathode, and at the front is a broad rectangular surface with colored phosphors on it. When the cathode is heated, it emits negatively charged electrons. Those electrons are attracted to the positively charged front of the CRT where they strike the phosphors and cause them to illuminate. One or more electron gun mechanisms are employed inside the monitor to direct the electrons precisely, so they don’t just fly around aimlessly. In a color CRT, there are three guns: one each for red, blue, and green.
Of course, the guns don’t emit colored electrons, so how do the colors get formed on the screen? For each pixel on the screen, there are three phosphors—one red, one blue, and one green—arranged in a triangle, or triad. Each electron gun works only on dots of a certain color. So, if a certain pixel is supposed to be purple, the red and blue guns would fire at that triad but not the green gun, and your eye would see purple. The distance between one color in a triad and the same color in the adjacent triad is known as the dot pitch of the monitor.
As there is great potential for misalignment error when you are dealing with such small phosphors, several technologies can keep the electron beams properly aligned. The most common is a shadow mask, a thin sheet of perforated metal that sits between the guns and the phosphors. Each gun directs itself through the designated hole for a particular triad, masking any stray electrons. Another technology for accomplishing the same thing is an aperture grille, which is made up of vertical wires between the guns and the phosphors. Still a third method, slot mask, is a combination of the two other technologies. In both aperture grille and slot mask, there are red, green, and blue stripes rather than interspersed dots running the length of the monitor. The main difference between those two is the number and placement of the stabilizing wires.
As soon as an electron hits a phosphor, it immediately begins to decay, so each triad in the display must be refreshed by a hit with the electron gun many hundreds of times per second. If pixels aren’t refreshed quickly enough, the display flickers, causing eyestrain. The measurement of how many times per second the display is refreshed is its refresh rate.
How LCD monitors work
The other common display type nowadays is a liquid crystal display, or LCD. You’ll find these in laptops and flat-panel workstation monitors.
An LCD screen has two polarized filters with liquid crystals between them. For light to appear on the display screen, it must pass through both filters and the crystals. The second filter, however, is at an angle to the first, so by default, nothing can pass through to the display. Applying a current to a crystal causes it to twist, which also twists the light passing through it. If the light twists so it matches the angle of the second filter, it can pass through it and light up an area of the display. On a color LCD, an additional filter splits the light into separate cells for red, green, and blue. Because there is no need for a mask to help direct electrons, there is less dark area between each pixel; that’s what gives an LCD display that saturated appearance that most CRTs cannot fully duplicate.
There are several different technologies for directing and controlling an LCD display. They differ primarily in the number of transistors controlling the cells. On a passive matrix display, there is one transistor for each row of cells and one transistor for each column of cells, much like spreadsheet row numbers and column letters. The transistors emit pulsing charges, and the combination of charges from two sides twists the liquid crystals in that row/column intersection. Since a passive matrix relies on pulsing, each pixel has moments when it is not receiving any signal; so passive matrix displays are not as bright as other types of LCD. For example, on a laptop with a maximum monitor resolution of 1,024 x 768, there are a total of 1,792 (1,024 + 768) transistors.
Double-scan passive matrix displays were developed to help improve the brightness of a passive matrix display. These use the same technology as a passive display but divide the screen into two sections with separate transistor rows/columns for each section. This allows the screen to be refreshed more rapidly.
One of the unfortunate qualities of any passive matrix screen is that it’s difficult to view at an angle. Looking at the screen straight-on gives a fairly bright image, but if several people are viewing the screen, anyone at an angle won't be able to see the screen as well as the person directly in front of it.
In contrast, active matrix LCD provides a separate transistor for each pixel. Since pixels don’t have to share, pulsing is not required and each cell can be constantly on. This results in a much brighter display that's visible from any angle, as well as a display that uses a lot more power. The latest type of active matrix display uses not one but three transistors for each pixel, resulting in very high refresh and redraw rates and even higher power consumption.
Not only should you know what goes on inside a monitor, but you should also understand common terms used by monitor manufacturers to help you in your troubleshooting endeavors. Here's a brief review.
This is the number of pixels that comprise the display. For example, a CRT might have a maximum resolution of 1,280 x 1,024. Thus, the CRT has 1,280 triads across and 1,024 triads down, for a total of 1,310,720 pixels. Most monitors can operate in a range of resolutions by treating several triads as an individual pixel. On a CRT, this is no big deal, and in fact, most people operate CRTs at a lower resolution than the maximum. However, LCD monitors are less adept at simulating lower resolutions than they are at simulating their maximum, so an LCD monitor’s display (especially text) can look fuzzy at lower resolutions. If set to a lower resolution, a high-resolution LCD monitor might actually give inferior display results.
Dot pitch or stripe pitch
As mentioned, the distance between two phosphors of the same color on a CRT is its dot pitch. A lower number means the dots are closer together, which makes for a better-quality picture. LCD monitors are also evaluated in dot pitch.
On an aperture grille or slot mask monitor, stripe pitch measures the distance from one colored stripe to another stripe of the same color. Again, lower is better. Some CRTs have a dot pitch or stripe pitch of about 0.28 mm, while the highest quality monitors have 0.22 mm or so.
The monitor’s top speed for refreshing each pixel in the display is its maximum refresh rate. The higher the resolution used, the more challenging it is for the monitor to refresh it at top speeds because of the increased number of unique pixels involved. At a refresh rate of lower than 75 Hz or so, a display flickers noticeably, so a high maximum refresh rate is an important feature in a monitor. The maximum refresh rate is typically not expressed as a single number, but rather as a separate number for each of several common resolutions. For example, a monitor might be capable of a 120-Hz refresh rate at 800 x 600 but only 85 Hz at 1,280 x 1,024.
Some older monitors have electron guns that cannot keep up with the refreshing needs to display a decent picture. Rather than spending more money for better electron guns, the manufacturers sometimes use a technique called interlacing to make the monitor’s picture flicker less. Interlacing refreshes only every other line of the display with each pass, rather than every line. Fortunately, monitor technology has advanced over the last few years to the point where interlacing is seldom necessary.
Testing a monitor
When a client complains about a monitor, the first thing I usually check is the video card. Often, display problems are actually video card problems, so before I assume the problem lies with the monitor, I check the following:
- Is the most recent version of the correct video driver installed in the OS? Is it the right version of the driver for that OS installed?
- Is the monitor snugly plugged into the video card, and is the video card snug in the motherboard?
- Does swapping out the video card solve the problem? If so, the problem is the video card.
- Does running Windows in Safe Mode solve the problem? If so, the problem is the video driver.
If looking for problems in an assumed-good display, switch to the highest resolution and refresh rate available. Any problems that are going to show up will show up at the maximum settings.
With all those potential causes eliminated, I follow these steps:
- Make sure the monitor has power. It may seem obvious, but it never hurts to look.
- Make sure the brightness and contrast are appropriately set.
- Adjust the controls on the monitor front or back. Sometimes, picture problems, such as a black ring around the display or a tilted or pincushion appearance, can be fixed from there.
- Try a different refresh rate. Sometimes, a too-high refresh rate can cause distortion or other problems. A too-low refresh rate will have users complaining about flickering, eyestrain, or overall poor-quality picture.
- If you have a monitor-testing program such as DisplayMate, run it. Some of these programs allow you to take a monitor through several resolutions and color depths; others are simply test patterns and solid color screens that you can use to visually check for display flaws.
- Display a pure white screen and look for convergence problems. This will appear as places on-screen with a red, green, or blue tint, and it indicates a problem with the dots in the triads not aligning properly. Some monitors have an adjustment to correct this. If you don’t have a utility that will give you a pure white screen, a shortcut is to open a new document in a word processing program and maximize the window. Also look for any individual pixels that are not white; this can indicate a damaged phosphor in that spot.
- If possible, display pure red, pure green, and pure blue screens and look for any dropped-out pixels. A monitor-testing program usually provides such screens.
While monitors suffering from damaged phosphors, convergence problems, or uncorrectable screen distortion are generally not worth fixing, most monitor issues can be corrected if you know what troubleshooting tips to apply. So now you’ll be prepared with an educated answer the next time someone asks “Why is my monitor doing that?”