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In today's high graphs outputs cards we can run into video problems very easily. In ordered to understand how these problems come about we must understand how a video card functions and the vast amount of data needed to display information on our monitors.
Understanding Graphics Accelerators
When screen resolutions approach or surpass 640x480, the data needed to form a single screen image can be substantial. Consider a single 640x480x256 image. There are (640x480) = 307,200 pixels. Because it has 256 colors, eight bits are needed to define the color for each pixel. Thus, 307,200 bytes are needed for every frame. If the frame must be updated 10 times per second, (307,200x10) = 3,072,000 bytes per second (3.072MB/sec) must be moved across the bus (i.e., PCI or ISA bus). If a 65,536 color mode is being used, two bytes are needed for each pixel, so (307,200x2) = 614,400 bytes are needed for a frame. At 10 frames per second, (614,400x10) = 6,144,000 bytes per second (6.144MB/sec) must be moved across the bus—this is just for video information and it does not consider the needs of system overhead operations, such as memory refresh, keyboard and mouse handling, drive access, and other data-intensive system operations. When such volumes of information must be moved across an ISA bus limited at 8.33 MHz, you can see how a serious data-transfer bottleneck develops. Even the PCI bus can be strained by higher video modes. This results in painfully slow screen refreshes—especially under Windows, which requires frequent refreshes.
Video designers seek to overcome the limitations of conventional video adapters by incorporating processing power onto the video board itself, rather than relying on the system CPU for graphic processing. By offloading work from the system CPU and assigning the graphics processing to local processing components, graphics performance can be improved by a factor of three or more. Several means of acceleration are possible, depending on the sophistication of the board.
A PC video system consists of four parts: the host PC itself, the video adapter/accelerator, the monitor, and the software (video BIOS and drivers). To deal with a failure in the video system, you must be able to isolate the problem to one of these four areas. When isolating the problem, your best tool is a working (or testbed) PC. With another PC, you can systematically exchange hardware to verify each element of the video system.
The computer is on, but there is no display
The PC seems to initialize properly. Be sure that the monitor is turned on and plugged into the video adapter properly. Also check that the monitor’s brightness and contrast controls are turned up enough (it sounds silly, but it really does happen). Try the monitor on a known-good PC. If the monitor works properly, suspect the video adapter. Power down the PC and be sure the video adapter is seated properly in its expansion slot. If any of the board contacts are dirty or corroded, clean the contacts by rubbing them with an eraser. You can also use any electronics-grade contact cleaner. You might want to try the video board in another expansion slot.
Chances are that the video adapter has at least one hardware jumper or DIP switch setting. Contact the manufacturer or refer to the owner’s manual for the board and check that any jumpers or DIP switch settings on the board are configured properly. If this is a new installation, check the adapter-board settings against the configuration of other expansion boards in the system. When the hardware settings of one board overlap the settings of another, a hardware conflict can result. When you suspect a conflict, adjust the settings of the video adapter (or another newly installed device) to eliminate the conflict. There might also be a memory conflict. Some video adapters make unusual demands of upper system memory (the area between 640KB and 1MB). It is possible that an Exclude switch must be added to the EMM386.EXE entry in a CONFIG.SYS file. Check with the adapter’s instruction manual to see if any memory configuration changes or optimizations are required.
Large, blank bands are at the top and bottom of the display in some
screen modes, but not in others
Multi-frequency and multi-mode monitors sometimes behave this way. This is not necessarily a defect, but it can cause some confusion unless you understand what is going on. When screen resolution changes, the overall number of pixels being displayed also changes. Ideally, a multi-frequency monitor should detect the mode change and adjust the vertical screen size to compensate (a feature called auto-sizing). However, not all multi-frequency monitors have this feature. When video modes change, you are left to adjust the vertical size manually. Of course, if information is missing from the display, a serious problem might be in the VRAM or the adapter’s graphics-controller IC. In this event, try another video adapter board.
The display image rolls
Vertical synchronization is not keeping the image steady (horizontal sync might also be affected). This problem is typical of a monitor that cannot display a particular screen mode. Mode incompatibility is most common with fixed-frequency monitors, but it can also appear in multi-frequency monitors that are being pushed beyond their specifications. The best course of action here is to simply reconfigure your software to use a compatible video mode (or reduce the vertical refresh rate). If that is an unsatisfactory solution, you will have to upgrade to a monitor that will support the desired video mode. If the monitor and video board are compatible, the problem is synchronization. Try the monitor on a known-good PC. If the monitor also fails on a known-good PC, try the known-good monitor on original PC. If the known-good monitor works on the suspect PC, the sync circuits in your original monitor have almost certainly failed. If the suspect monitor works on a known-good PC, the trouble is likely in the original video adapter. Try replacing the video adapter.
When returning to Windows from a DOS application, the Windows screen “splits” from top-to-bottom This is a DOS problem that is seen under Windows, which indicates an obsolete or corrupted video driver (for example, using a Windows 3.0 video driver under Windows 3.1). Chances are that the video adapter is running just fine. Be sure that the proper DOS “grabber” file is installed and specified in the SYSTEM.INI file. Check with the video-board manufacturer to obtain the latest assortment of drivers and grabber files. Try reinstalling the drivers from their master disk. If you do not have current drivers available, try switching to the generic VGA driver.
The system hangs up during initialization, some characters might be missing
from the display, or the screen colors might be incorrect
These are classic symptoms of a hardware conflict between the video adapter and one or more cards in the system, or an area of memory. Some video boards use an area of upper memory that is larger than the “classic” video area. For example, the Impact SVGA board imposes itself on the entire address range between A0000h and DFFFFh. In this kind of situation, any other device using an address in this range will conflict with the video board. A conflict might occur when the video board is first installed, or the board might work fine until another device is added or modified. Resolving a hardware conflict basically means that something has to give—one of the conflicting elements (i.e., IRQ lines, DMA channels, or I/O addresses) must be adjusted to use unique system resources. As a technician, it rarely matters which of the conflicting devices you change, but remember that system startup files, device drivers, and application settings might also have to change to reflect newly selected resources. You might also be able to resolve some memory conflicts by adding the EXCLUDE switch to EMM386.EXE. The video adapter manual will indicate when an EXCLUDE switch is necessary.
The system hangs up using a 16-bit VGA board and one or more 8-bit controllers
This is typically a problem that arises when 8-bit and 16- bit ISA boards are used in the same system. Because of the way that an ISA bus separates the 8-bit and 16-bit segments, accessing an 8-bit board when 16-bit boards are in the system might cause the CPU to (falsely) determine that it is accessing a 16-bit board. When this occurs, the system will almost invariably crash. Try removing any 8-bit boards from the system. If the crashes cease, you have probably nailed down the error. Unfortunately, the only real correction is to either remove the 8-bit board(s) or reconfigure the board(s) to use a higher area of memory.
You have trouble sizing or positioning the display, or you see error
messages, such as “Mode not supported” or “Insufficient memory”
These kinds of errors might occur in newer or high-end video boards if the board is not set up properly for the monitor it is being used with. Most new video boards include an installation routine that records the monitor’s maximum specifications, such as resolution (and refresh frequencies), horizontal scanning frequencies, and vertical scanning frequencies. If such data is entered incorrectly (or the monitor is changed), certain screen modes might no longer work properly. Check the video adapter’s installation parameters and correct its setup, if necessary.
Text appears in an odd color
For example, text that should be green appears black. This is almost always the result of a problem with the palette decoding registers on the particular video board, and will typically appear when using higher color modes (e.g., 64k or 16M colors). Be sure that the video drivers are correct, complete, and up-to-date. If the problem persists, you might need to replace the video board outright.
When an application is started (under Windows), the opening display appears
Although this might appear to be a video memory problem at first glance, it is actually more likely to be related to a buggy video driver. Upgrade the video driver to the latest version or try a generic video driver that is compatible with your video chipset.
The .AVI files have distorted colors or “grainy” playback
This usually occurs when playing 8-bit .AVI files that are not supported by DCI, and can usually be corrected by disabling the accelerated video playback features of the video board.
Boot problems occur after a new video board has been installed
Typical problems include no video or eight beeps when the system is turned on. This is usually the result of an outdated system BIOS, which is not capable of detecting the particular video chipset in use. The BIOS interprets this as meaning that no video board is in the system, and an error is generated accordingly. Contact the motherboard manufacturer (or PC maker) for an updated system BIOS. Most BIOS versions dated after the fall of 1994 should be able to detect most modern video chipsets.
Errors appear, such as “Insufficient video memory”
Not enough video memory is on the board to handle screen images at the resolution and color depth you have selected. In most cases, the system might crash outright. Your immediate solution should be to select a lower resolution or a smaller color palette. If you are encountering such problems when attempting to play .AVI or MPEG files, you should be able to select smaller video windows and lower color depth without altering your Windows setup. As a more long-term solution, you should consider adding more video memory or replacing the video board with one that contains more video memory.
Other devices don’t work properly after the PCI video card is installed
For example, the sound card output is distorted or a fast modem loses data. This can happen often with newer video adapters. Some computers require that software wait for the hardware to be ready to receive new data. Newer video board drivers are not normally set do this because it slows them down slightly (and it’s not necessary for most current computers). Under Windows , right-click on the Windows desktop background. Click the Properties menu item, select the video board’s Settings tab. Select the Advanced button, then click the Performance tab. Clear the “Use automatic PCI bus retry” check box. Finally, accept your changes and reboot the computer when instructed to do so. Under Windows 3.1x, edit the SYSTEM.INI file in your directory to add the line PciChipSet=1 to the particular video board’s section (e.g., [mga.drv]).
A Windows game doesn’t start or runs slower than normal
The program uses the Microsoft DirectX interface. DirectX might not be installed or an older version of DirectX is installed. Most programs that use DirectX install it as part of their installation, but some do not. Also, some older programs might install an earlier version of DirectX (overwriting a later version). To see if DirectX is installed:
If the current version of DirectX is installed, you’re finished. Otherwise, you’ll need to install DirectX: If the DirectX setup program asks if you want to replace the existing display drivers, click No.
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