Before you start
Objectives: Learn what is CRT monitor, how does it work and what are its features.
Key terms: screen, color, refresh, phosphor, rate, gun, electron, green, blue, pixel, red, draw, display, horizontal, vertical, resolution, grill, mask, intensity, video, aperture
CRT Monitor Design
A CRT monitor is made up of several different components. The first component is the phosphor screen that we see when we’re using a computer. The phosphor screen is attached to one end of a vacuum tube. When we say vacuum tube we mean that all the air has been pumped out of the tube. At the back of the vacuum tube is a big magnet and within the magnet there is an electron gun. Signal coming from source such as a video card is applied to the electron gun. When that happens the electron gun shoots a stream of electrons that hit the phosphor screen. Whenever a phosphor in the phosphor screen is hit by an electron it glows for just a short period of time. Electron gun is aimed by the magnet that we mentioned before. Current is applied to the magnet, changing its magnetism which causes the electron gun to move so that it shoots electrons across the whole phosphor screen. Even though all the phosphors aren’t lit up at the same time, we see the whole picture. Electron gun goes so fast back and forth that to us it looks like a solid picture. Our eyes are not fast enough to notice this drawing of the screen. A modern VGA monitor actually uses three different electron guns. It uses a red, a green, and a blue electron gun. Each of these guns is aimed at a red, green or blue phosphor in the screen.
With a monochrome monitor each phosphor is a single dot. This dot can be varied in intensity, hence we’re able to create a grayscale image using a series of single, monochrome phosphors. To darken a phosphor dot we apply less electrons to it. If we want to have a lighter phosphor we apply a high intensity electron stream to it.
Image 321.2 – Gray scale Example
By doing this we’re able to create a series of grey scales from white to black, and when applied to the screen we’re able to create an image. This is how a monochrome monitor works. With a color monitor each phosphor is made up of three smaller phosphors. We have a red, a green, and a blue phosphor.
Image 321.3 – RGB
By varying the intensity applied to each of the dots in the phosphor we’re able to create all kinds of different colors. For example, to create white color we apply a high intensity electron stream to all three phosphors at the same time. To create a black dot we would not apply any, or a very low intensity to the three phosphors. If we wanted to create a color in between white or black we vary the intensity of each color. Because of those three color dots we actually have to use three different electron guns in the vacuum tube. We use a red, a green and a blue gun. Each gun is aimed at a specific phosphor.
Shadow Mask and Aperture Grill
Since three phosphor dots are put very close together, we have to ensure that the right gun hits the right color. To do that we use a shadow mask or aperture grill. A shadow mask is a metal screen full of holes that are aligned with each phosphor dot. Its job is to prevent a particular electron gun from hitting the wrong color. So the red gun can only hit the red color, the green gun the green color and the blue gun the blue color. If something goes wrong and the gun misses its hole, it will not accidentally hit the wrong color, in fact it won’t hit any color. An aperture grill is a set of ultra thin vertical and horizontal wires. This configuration allows a richer color display because it lets more electrons through. On an aperture grill monitor we might see two horizontal lines on the display. These lines are the wires that are used to dynamically change the grill opening.
To draw a screen electron guns start at the top left corner, move across the screen and draw a single line, then they come down, draw another line until it makes a full pass from top to bottom, drawing the full screen. This happens very fast so we don’t see it, but we have to remember that that screen is constantly being redrawn. This brings us to the refresh rate parameter of CRT monitors. There are two different parameters when talking about the refresh rate. We differentiate the horizontal refresh rate and the vertical refresh rate. The horizontal refresh rate is the speed it takes to draw a single line across the screen from left to right. The vertical refresh rate is the speed it takes to draw the entire screen, from the top to the bottom corner of the screen and then return to the top corner. Early CRT monitors had a fixed refresh rate so we had to make sure that the refresh rate on our video board matches the refresh rate of our CRT monitors. If we pushed our monitor over its refresh rate threshold we could actually destroy it. Today, it’s not a big issue because monitors and video boards are capable of running at different refresh rates. The refresh rate is determined by the video board, not the monitor. On the other hand, if we set the refresh rate to low, it could case eye strain and headaches because the screen is drawn to slowly. So we should always check the maximum refresh rate that our monitor supports and then configure the video card to use that particular rate. If the refresh rate is to high, we could damage our monitor. Refresh rates are measured in Hz.
Resolution is the number of horizontal and vertical pixels displayed on the screen. Pixel is short for picture element. Pixel is a dot on the screen composed of three phosphors (red, green, blue). However a pixel can also be multiple dots on the screen. The more dots we include in a single pixel, the lower the resolution is on the monitor. That’s because we’re using up larger areas of the monitor screen to represent a single pixel. On a CRT monitor pixels are arranged in a 4:3 aspect ratio, which matches the display ration of a television set. That means the the number of pixels on the top (width) has to be multiple of four, and the number of pixels on the side (height) has to be multiple of three. Common resolutions adhering to 4:3 aspect ratio are 640 by 480, 800 by 600, 1024 by 768, 1280 by 1024, etc. Remember that when we use a lower resolution we’re actually including more phosphors on the screen in a single pixel. Monitors that support a higher resolution can display higher-quality graphics or have a larger screen area.
The dot pitch on a CRT monitor is the distance between phosphors of the same color on the screen. It is measured in millimeters. The ranges go from 0.30 millimeters down to 0.15 millimeters.
Image 321.4 – Dot Pitch
The closer those phosphor dots are, the higher the definition of the picture is. Monitors with higher definition are also more expensive because of the higher picture quality.
If select a refresh rate that our monitor can’t handle or if we select a resolution that’s too high for the monitor to support, the monitor will try to compensate that by using interlacing. With interlacing the monitor draws half of the lines in a single pass. It takes two passes to draw the entire screen. On the first pass it draws the odd number lines, then it comes back and in a second pass draws the even numbered lines. The problem with interlacing is that it can cause eye strain and headaches if we look at it for too long, but it will prevent prevent the monitor from being damaged. Non-interlaced monitors produce the least amount of flicker.
Color depth is the number of colors that can be displayed simultaneously on the screen. Different color depth require different amount of memory. The formula for maximum number of colors that can be displayed simultaneously is 2 to the power of the number of bits per pixel.
For example, if we use 8 bits per pixel, the number would be 2 to the power of 8, which is 256 colors on the screen at the same time. On modern systems use 24 bit color (16.7 million different colors on the screen), or even 32 bit color representation (4.2 billion different possible colors that can be displayed simultaneously on the screen). 32 bit color is considered true color.
Screen size is sometimes expressed in two different measures. The physical screen size (the actual size) is the diagonal measure of the display tube. The effective viewing size is the largest image size that can be displayed. Portions of the display tube cannot be used for displaying the image because of how the CRT monitor works.
Virtually all CRT monitors connect to the video card using a DB-15 (also called a D-sub) connector, but it is possible for a CRT monitor to have a digital connection. Monitors that are Energy Star or Green Star compliant use less than 30 watts and reduce power consumption by 99% when in sleep or suspended mode. CRT monitors can have flat displays (tube monitors with a flatter glass tube). A flatter display has an image with less distortion around the edges. The magnetic field used for drawing the image on the screen can cause a buildup of magnetism on internal monitor components. This magnetic field can cause color distortions, especially around the edges. To correct this problem, we use the degauss feature of the monitor. Degaussing creates an oscillating magnetic field for a short period of time. Most monitors perform degaussing automatically when they are turned on, but we can also manually initiate degaussing using the monitor’s front panel controls.
A modern VGA monitor uses three different electron guns: a red, a green, and a blue electron gun. Each of these guns is aimed at a red, green or blue phosphor in the screen. We have to ensure that the right gun hits the right color and we do that using a shadow mask or aperture grill. The horizontal refresh rate is the speed it takes to draw a single line across the screen from left to right. The vertical refresh rate is the speed it takes to draw the entire screen, from the top to the bottom corner of the screen and then return to the top corner. Refresh rates are measured in Hz. Resolution is the number of horizontal and vertical pixels displayed on the screen. Pixel is short for picture element. CRT monitors have a 4:3 aspect ratio. Common resolutions adhering to 4:3 aspect ratio are 640 by 480, 800 by 600, 1024 by 768, 1280 by 1024, etc. The dot pitch on a CRT monitor is the distance between phosphors of the same color on the screen. It is measured in millimeters. Interlacing is drawing the screen in two passes. Odd lines are drown on the first and then the even lines on the second pass. Color depth is the number of colors that can be displayed simultaneously on the screen.