43 Ghosts, shadows, or streaks in picture adjacent to vertical edges
Description
This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
43 Ghosts, shadows, or streaks in picture adjacent to vertical edges
Complaints about these kinds of problems are very common especially as the screen resolution and necessary video bandwidth keeps increasing. Most are due to cable and video termination deficiencies and not actual monitor defects.
The video signals for red, green, and blue (or just a single signal for monochrome) are sent over cables which are generally 75 ohm transmission lines. These are coaxial cables that may be combined inside a single sheath for VGA, SVGA, Macs, and many workstations but may be separate coaxes with BNC (or other) connectors for other video applications.
Without going into transmission line theory, suffice it to say that to obtain good quality video, the following conditions must be met:
1. A good quality of cable must be used. This means one in which the characteristic impedance is close to the optimum 75 ohms, one which has low losses, and one which has good shielding. For installations using BNC connectors, a good quality of 100% shielded RG59U is often used. The BNC connectors must be properly installed or they will contribute to mismatch problems.
2. Where multiple monitors are to be connected to a single video source, all wiring is done in a daisy chain fashion. The only taps permitted are the minimum necessary to connect each monitor to the chain. This usually means a BNC-T connector or a pair of connectors on the monitor for each video signal. T connections with cable must be avoided.
3. Only the last monitor in the chain should be terminated in 75 ohms. All of the others must be set to Hi-Z. Monitors with BNC connectors will usually have one switch or a switch for each color to select termination.
Monitors for PCs, Macs, and workstations usually have built in termination and do not offer the choice of Hi-Z. This means that without a video distribution amplifier, it is not possible to connect multiple monitors of this type to a single video source with any expectation of a good quality display.
Failure to follow these rules will result in video ringing, ghosts, shadows, and other unsightly blemishes in the picture. It is often not possible to control all aspects of the video setup. The cable is often a part of the monitor and cannot easily be substituted for a better one. The monitor may not have properly designed circuitry such that it degrades the video regardless of the cable and display board quality. The display card itself may not have proper drivers or source termination.
Ironically, the better the video card, the more likely that there will be visible problems due to termination. This is due to the very high bandwidth and associated signal edge rates.
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