Help Desk.How It Works.Page Five

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Monitors and VRAM

Although computer monitors use many of the same components found inside televisions, there is one LARGE difference. Computer monitors are DIGITAL devices, which means they display images with more clarity and color accuracy than even the best televisions.

The Basics: That big glass screen in the front of a device called a CRT (Cathode Ray Tube). In the back of the CRT is a "gun" that shoots out a stream of negatively charged electrons. A device near the front of the CRT (called a "flyback transformer") creates a positive charge that pulls the electrons toward the front of the tube until they smash into the PHOSPHORS (light-emitting chemicals) on the inside of the glass screen. As each phosphor is hit by electrons, it glows briefly and you see a spot of light on your display. Got it?

Let's get a little more technical: In a color monitor, there are three different kinds of phosphors: One type glows Red, another Green, and the third glows Blue. These phosphors are arranged on the inside of the glass screen in small groups called PIXELS (short for "Picture Elements"). Each pixel contains three phosphors, one for each color.

In addition, color monitors use three different electron "guns" to shoot electrons at the Red, Green and Blue phosphors individually. But, the three phosphors that make up each pixel are so small your brain can't tell them apart. Instead, it blends the glow of the phosphors together into ONE point of light. If none of the phosphors are glowing, the pixel appears to be black. If all three phosphors are at full intensity, the colors will mix and the pixel appears to be white. And, through the magic of physics, millions of additional colors can be created by varying the individual brightness of each pixel's red, green and blue phosphors.

Now for the REALLY technical stuff: When a phosphor is hit by electrons, it only glows for a brief moment. So, the electrons need to smash into the phosphors on a fairly regular basis. How often? Well, that clear, steady image on your screen right now is actually being "refreshed" (completely erased and recreated) at least 60 times each second! Since that's faster than your brain can figure it out, you think it's one steady image. (NOTE: The actual number of screen refreshes each second ranges from 60-102, depending on the monitor.)

In order to hit every phosphor on a regular basis, the electron streams move around inside the monitor in a consistent pattern. Each stream starts in the upper left corner and travels across the screen in one thin horizontal line. It then turns itself off, goes back to the left side of the screen, moves down one line, turns back on, and "scans" across a second horizontal line. Then it turns off again, goes back to the beginning of the third line, turns back on, and continues scanning across each individual line until it reaches the bottom right corner of the screen. At that point, the stream turns off, returns to the upper left corner, turns back on, and runs through the whole pattern again. Left to right, top to bottom, the electron streams hit every single phosphor on the screen at least 60 times each second.

Meanwhile, in the back of the CRT are a collection of electromagnets. They control the vertical and horizontal movement of the electron streams using synchronization (sync) signals provided by the computer. (The sync signals tell the CRT when to turn the electron streams on or off, where the stream should be aimed, and how often the pattern needs to be repeated.)

So, why is all this technical stuff important? If your monitor ever develops a problem, understanding how it creates an image may help you determine what's wrong. For example, three of the most common monitor problems are:

Screen image suddenly turns blue, red or green: One or more of the electron "guns" isn't working. (Example: If the screen has a blue-green tint, the red gun isn't firing its electrons correctly.)

Screen is filled with "snow," or the image is a mixed up mess of flickering lines. Either your computer is sending the wrong type of "sync signals," or your monitor has a problem that prevents it from decoding the signals being sent.

Screen image is black except for a single white line. One of the magnets that control the electron streams isn't working. (Example: If the magnet that controls vertical movement fails, the electron stream will constantly move back and forth in a single horizontal line - creating a bright, white streak across the middle of a blank screen.)

What you see on the screen: The first thing you need to realize is that the Macintosh "desktop" isn't limited to what you see. The desktop is a HUGE grid that measures 65,000 spaces tall, and 65,000 spaces wide. Your monitor is like a little "window," through which you can see a small part of the full desktop. Of course, you can see more of the desktop through a 21" (diagonal) monitor "window" than you can through an older 14" monitor. But, the size of the glass screen is only part of the equation. The real measure of your "window" to the desktop is defined by the monitor's "resolution."

Monitor resolution is measured in pixels, and defines the portion of the full desktop "grid" you can see as being a space "X pixels wide by X pixels tall." For example, older 14" monitors always displayed a small portion of the desktop, measuring only 640 pixels wide and 480 pixels tall. Most modern monitors, however, have "multisync" capabilities. That means YOU get to define the specific size of your monitor's "window to the desktop" (up to 1600 x 1200 pixels), regardless of the actual size of the glass.

Hold on there! Don't confuse the glowing phosphor pixels inside your monitor with the number of "desktop pixels" you choose to display. The glowing pixels never move. They are always in the same place, arranged in neat rows all across your monitor screen. (The spaces between the pixels is measured in millimeters, and ranges from 0.25mm to 0.31mm. This number is known as the "dot pitch." Lower numbers mean that the pixels are closer together and will, therefore, display an image with finer detail.)

What actually changes when you vary the resolution of a multisync monitor is the number of desktop "grid spaces" represented by the glowing screen pixels. For example, a 15" multisync monitor usually displays a portion of the desktop measuring 832 x 624 pixels. But, if your eyesight is good, you might change it to display an area that measures 1,024 x 768 pixels. To do this, the computer "shrinks' the defined portion of the desktop grid until it fits your monitor screen. (NOTE: Displaying large resolutions on small monitors makes everything appear to be smaller, so text will be very hard to read.)

And now, finally, we get to the point.

Aside from the physical limitations of the monitor itself, there are two factors which limit the maximum resolution you can view on screen: The amount of VRAM (Video RAM, pronounced Vee-RAM) in your computer, and the "Color Depth" you select.

VRAM is just like regular RAM (a storage space for instructions), except that it only stores the instructions needed to display the screen image. Some older Macs didn't have separate VRAM. The regular RAM handled the monitor image instructions, too. But with separate VRAM installed, those instructions are "off-loaded," which makes the whole machine run a little faster.

How much VRAM do you need? Well, that depends on the "Color Depth" you select. The color depth (Bit Depth) determines how many BITS of data are used to define the color of each desktop pixel.

If you open your Monitors and Sound Control Panel, you'll see it contains two little windows. One lists the various "resolutions" you have available. (If your monitor isn't a "multisync" model, you'll only see one resolution.) The other window contains a selection of color settings.

Depending on your system, you can choose between "grayscale" or color palettes which include "Black and White" (1 bit of data is used to define the color of each pixel, it's either ON or OFF), 4 colors or shades of gray (2 bits per pixel), 16 colors or grays (4 bits per pixel), and 256 colors or grays (1 BYTE per pixel, also known as "8-bit color").

In addition, some systems may offer two other selections for color monitors: "Thousands" of colors (more than 32,000 variations) and "Millions" of colors (16,777,216 to be exact - more than the human eye can actually see!) These two choices are often referred to as "16-bit color" (2 Bytes per pixel) and "24-bit color" (3 Bytes per pixel). (NOTE: Some scanners claim to be able to recognize BILLIONS of colors by defining each pixel using four Bytes of data. You may see this referred to as 30-bit or 32-bit color.)

Are you ready for the really, really important news? OK, here it is: Color Depth, Resolution and VRAM size are all related!

To understand how they work together, let's imagine you have a 15" multiscan Monitor and 1 MB of VRAM installed in your computer. If you choose the standard screen resolution of 832 x 624, your monitor will display a total of 519,168 desktop pixels on screen (Multiply the resolution width times height to determine the total number of pixels on screen.) Now, let's set the color depth to 256, which defines the color of each pixel using one Byte (8 bits) of data. The result? To maintain the monitor image, the VRAM will need to send 519,168 Bytes of data to the monitor for every screen redraw - AT LEAST 60 TIMES EACH SECOND. That's a lot of data!

What happens if we select "Thousands" of colors? The computer needs to send twice as much information, two Bytes of data for each pixel. In our example, that would be a total of 1,038,336 Bytes for the whole screen. Now we have a problem! Your Mac's 1 MB of VRAM (1,048,576 Bytes) isn't enough to hold both the color information AND the basic "how to draw a screen" instructions at the same time. So, for you, "Thousands" of colors won't be an option. And displaying "Millions" of colors (requiring 1,557,504 Bytes of data per screen refresh) would be completely out of the question! But, all is not lost. You do have an option. Actually, you have two. Wait, make that three!

Add more VRAM. (As the example above shows, 2 MB will easily handle "Thousands" or "Millions" of colors at a resolution of 832 x 624.)

Reduce the screen resolution. The next smaller Mac standard screen resolution is 640 x 480 pixels. That's only 307,200 pixels per screen. And at 2 Bytes per pixel (for "Thousands" of colors) you would only need 614,400 Bytes of VRAM - well within your 1 MB limit. You might also have the PC standard 800 x 600 resolution available. That creates 480,000 pixels on screen. If you want "Thousands" of colors (960,000 Bytes of color data), you might be able to squeak by with only 1 MB of VRAM.

Buy a graphics accelerator. A graphics accelerator is a plug-in card that contains it's own VRAM, and its own microprocessor(s), to take the burden of refreshing your screen off the CPU. Cheap accelerators contain 2 MB of VRAM. The best have up to 8 MB of VRAM, and additional processors to handle 3D and multimedia tasks. (NOTE: Adding a graphics card opens up the option of using a "Dual-Monitor" system. Visit Page 4 of our "Practical Mac" area for more details.)

One final thought. Really! This is it!

To most of us, "more is better." We always want to display the largest resolution and widest range of colors possible. Unfortunately, everything has its price. And, in this area, the price is a loss of performance.

We've investigated dozens of customer complaints about "slow" computers, only to find that the computer itself is speeding along just fine. What's killing them is painfully slow scrolling and screen redraws caused by setting the resolution and bit depth too close to the VRAM's limits. (NOTE: Your Mac may need to refresh the screen in smaller, more manageable blocks - very slowly, one at a time - if it doesn't have enough VRAM to handle the whole screen at once.)

Our advice? Do yourself a favor. Set your monitor's bit depth to 256 colors, and leave it there whenever possible. (NOTE: For more details about this advice, visit Page 8 of our "Practical Mac" area.)

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Help: How it Works: 5 of 9
Monitors and VRAM Although computer monitors use many of the same components found inside televisions, there is one LARGE difference. Computer monitors are DIGITAL devices, which means they display images with more clarity and color accuracy than even the best televisions. The Basics: That big glass screen in the front of a device called a CRT (Cathode Ray Tube). In the back of the CRT is a "gun" that shoots out a stream of negatively charged electrons. A device near the front of the CRT (called a "flyback transformer") creates a positive charge that pulls the electrons toward the front of the tube until they smash into the PHOSPHORS (light-emitting chemicals) on the inside of the glass screen. As each phosphor is hit by electrons, it glows briefly and you see a spot of light on your display. Got it? Let's get a little more technical: In a color monitor, there are three different kinds of phosphors: One type glows Red, another Green, and the third glows Blue. These phosphors are arranged on the inside of the glass screen in small groups called PIXELS (short for "Picture Elements"). Each pixel contains three phosphors, one for each color. In addition, color monitors use three different electron "guns" to shoot electrons at the Red, Green and Blue phosphors individually. But, the three phosphors that make up each pixel are so small your brain can't tell them apart. Instead, it blends the glow of the phosphors together into ONE point of light. If none of the phosphors are glowing, the pixel appears to be black. If all three phosphors are at full intensity, the colors will mix and the pixel appears to be white. And, through the magic of physics, millions of additional colors can be created by varying the individual brightness of each pixel's red, green and blue phosphors. Now for the REALLY technical stuff: When a phosphor is hit by electrons, it only glows for a brief moment. So, the electrons need to smash into the phosphors on a fairly regular basis. How often? Well, that clear, steady image on your screen right now is actually being "refreshed" (completely erased and recreated) at least 60 times each second! Since that's faster than your brain can figure it out, you think it's one steady image. (NOTE: The actual number of screen refreshes each second ranges from 60-102, depending on the monitor.) In order to hit every phosphor on a regular basis, the electron streams move around inside the monitor in a consistent pattern. Each stream starts in the upper left corner and travels across the screen in one thin horizontal line. It then turns itself off, goes back to the left side of the screen, moves down one line, turns back on, and "scans" across a second horizontal line. Then it turns off again, goes back to the beginning of the third line, turns back on, and continues scanning across each individual line until it reaches the bottom right corner of the screen. At that point, the stream turns off, returns to the upper left corner, turns back on, and runs through the whole pattern again. Left to right, top to bottom, the electron streams hit every single phosphor on the screen at least 60 times each second. Meanwhile, in the back of the CRT are a collection of electromagnets. They control the vertical and horizontal movement of the electron streams using synchronization (sync) signals provided by the computer. (The sync signals tell the CRT when to turn the electron streams on or off, where the stream should be aimed, and how often the pattern needs to be repeated.) So, why is all this technical stuff important? If your monitor ever develops a problem, understanding how it creates an image may help you determine what's wrong. For example, three of the most common monitor problems are: Screen image suddenly turns blue, red or green: One or more of the electron "guns" isn't working. (Example: If the screen has a blue-green tint, the red gun isn't firing its electrons correctly.) Screen is filled with "snow," or the image is a mixed up mess of flickering lines. Either your computer is sending the wrong type of "sync signals," or your monitor has a problem that prevents it from decoding the signals being sent. Screen image is black except for a single white line. One of the magnets that control the electron streams isn't working. (Example: If the magnet that controls vertical movement fails, the electron stream will constantly move back and forth in a single horizontal line - creating a bright, white streak across the middle of a blank screen.) What you see on the screen: The first thing you need to realize is that the Macintosh "desktop" isn't limited to what you see. The desktop is a HUGE grid that measures 65,000 spaces tall, and 65,000 spaces wide. Your monitor is like a little "window," through which you can see a small part of the full desktop. Of course, you can see more of the desktop through a 21" (diagonal) monitor "window" than you can through an older 14" monitor. But, the size of the glass screen is only part of the equation. The real measure of your "window" to the desktop is defined by the monitor's "resolution." Monitor resolution is measured in pixels, and defines the portion of the full desktop "grid" you can see as being a space "X pixels wide by X pixels tall." For example, older 14" monitors always displayed a small portion of the desktop, measuring only 640 pixels wide and 480 pixels tall. Most modern monitors, however, have "multisync" capabilities. That means YOU get to define the specific size of your monitor's "window to the desktop" (up to 1600 x 1200 pixels), regardless of the actual size of the glass. Hold on there! Don't confuse the glowing phosphor pixels inside your monitor with the number of "desktop pixels" you choose to display. The glowing pixels never move. They are always in the same place, arranged in neat rows all across your monitor screen. (The spaces between the pixels is measured in millimeters, and ranges from 0.25mm to 0.31mm. This number is known as the "dot pitch." Lower numbers mean that the pixels are closer together and will, therefore, display an image with finer detail.) What actually changes when you vary the resolution of a multisync monitor is the number of desktop "grid spaces" represented by the glowing screen pixels. For example, a 15" multisync monitor usually displays a portion of the desktop measuring 832 x 624 pixels. But, if your eyesight is good, you might change it to display an area that measures 1,024 x 768 pixels. To do this, the computer "shrinks' the defined portion of the desktop grid until it fits your monitor screen. (NOTE: Displaying large resolutions on small monitors makes everything appear to be smaller, so text will be very hard to read.) And now, finally, we get to the point. Aside from the physical limitations of the monitor itself, there are two factors which limit the maximum resolution you can view on screen: The amount of VRAM (Video RAM, pronounced Vee-RAM) in your computer, and the "Color Depth" you select. VRAM is just like regular RAM (a storage space for instructions), except that it only stores the instructions needed to display the screen image. Some older Macs didn't have separate VRAM. The regular RAM handled the monitor image instructions, too. But with separate VRAM installed, those instructions are "off-loaded," which makes the whole machine run a little faster. How much VRAM do you need? Well, that depends on the "Color Depth" you select. The color depth (Bit Depth) determines how many BITS of data are used to define the color of each desktop pixel. If you open your Monitors and Sound Control Panel, you'll see it contains two little windows. One lists the various "resolutions" you have available. (If your monitor isn't a "multisync" model, you'll only see one resolution.) The other window contains a selection of color settings. Depending on your system, you can choose between "grayscale" or color palettes which include "Black and White" (1 bit of data is used to define the color of each pixel, it's either ON or OFF), 4 colors or shades of gray (2 bits per pixel), 16 colors or grays (4 bits per pixel), and 256 colors or grays (1 BYTE per pixel, also known as "8-bit color"). In addition, some systems may offer two other selections for color monitors: "Thousands" of colors (more than 32,000 variations) and "Millions" of colors (16,777,216 to be exact - more than the human eye can actually see!) These two choices are often referred to as "16-bit color" (2 Bytes per pixel) and "24-bit color" (3 Bytes per pixel). (NOTE: Some scanners claim to be able to recognize BILLIONS of colors by defining each pixel using four Bytes of data. You may see this referred to as 30-bit or 32-bit color.) Are you ready for the really, really important news? OK, here it is: Color Depth, Resolution and VRAM size are all related! To understand how they work together, let's imagine you have a 15" multiscan Monitor and 1 MB of VRAM installed in your computer. If you choose the standard screen resolution of 832 x 624, your monitor will display a total of 519,168 desktop pixels on screen (Multiply the resolution width times height to determine the total number of pixels on screen.) Now, let's set the color depth to 256, which defines the color of each pixel using one Byte (8 bits) of data. The result? To maintain the monitor image, the VRAM will need to send 519,168 Bytes of data to the monitor for every screen redraw - AT LEAST 60 TIMES EACH SECOND. That's a lot of data! What happens if we select "Thousands" of colors? The computer needs to send twice as much information, two Bytes of data for each pixel. In our example, that would be a total of 1,038,336 Bytes for the whole screen. Now we have a problem! Your Mac's 1 MB of VRAM (1,048,576 Bytes) isn't enough to hold both the color information AND the basic "how to draw a screen" instructions at the same time. So, for you, "Thousands" of colors won't be an option. And displaying "Millions" of colors (requiring 1,557,504 Bytes of data per screen refresh) would be completely out of the question! But, all is not lost. You do have an option. Actually, you have two. Wait, make that three! Add more VRAM. (As the example above shows, 2 MB will easily handle "Thousands" or "Millions" of colors at a resolution of 832 x 624.) Reduce the screen resolution. The next smaller Mac standard screen resolution is 640 x 480 pixels. That's only 307,200 pixels per screen. And at 2 Bytes per pixel (for "Thousands" of colors) you would only need 614,400 Bytes of VRAM - well within your 1 MB limit. You might also have the PC standard 800 x 600 resolution available. That creates 480,000 pixels on screen. If you want "Thousands" of colors (960,000 Bytes of color data), you might be able to squeak by with only 1 MB of VRAM. Buy a graphics accelerator. A graphics accelerator is a plug-in card that contains it's own VRAM, and its own microprocessor(s), to take the burden of refreshing your screen off the CPU. Cheap accelerators contain 2 MB of VRAM. The best have up to 8 MB of VRAM, and additional processors to handle 3D and multimedia tasks. (NOTE: Adding a graphics card opens up the option of using a "Dual-Monitor" system. Visit Page 4 of our "Practical Mac" area for more details.) One final thought. Really! This is it! To most of us, "more is better." We always want to display the largest resolution and widest range of colors possible. Unfortunately, everything has its price. And, in this area, the price is a loss of performance. We've investigated dozens of customer complaints about "slow" computers, only to find that the computer itself is speeding along just fine. What's killing them is painfully slow scrolling and screen redraws caused by setting the resolution and bit depth too close to the VRAM's limits. (NOTE: Your Mac may need to refresh the screen in smaller, more manageable blocks - very slowly, one at a time - if it doesn't have enough VRAM to handle the whole screen at once.) Our advice? Do yourself a favor. Set your monitor's bit depth to 256 colors, and leave it there whenever possible. (NOTE: For more details about this advice, visit Page 8 of our "Practical Mac" area.)
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HOME . SERVICES . HELP DESK . RESOURCES . WHY MAC . CREDENTIALS
Copyright 1998, 5-Minute Mac Consulting, Wampum, Pennsylvania, USA