Wednesday, November 23, 2011

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.

Some examples of common termination problems:

* Overly bright picture with trails following vertical edges, perhaps with
periodic ringing. This is due to a missing termination. Check if the
monitor is set for Hi-Z instead of 75 ohms. If there is no switch, then
the termination may be faulty or the monitor may need an external resistor.
For BNC connectors, plug-on terminations are available.

* Bright ghost images adjacent to vertical lines. This may indicate that
the terminating resistor is greater than the impedance of the cable.
You may be using Ethernet Thinnet cable by accident which is RG58 with
an impedance of 50 ohms.
* Dark picture and ghost images adjacent to vertical lines. This may indicate
that the terminating resistor is too low - multiple monitors on a chain all
set for 75 ohms instead of just the last one. Or, an improper type of cable
such as audio patch cord.

* Fuzzy vertical edges. This may indicate a poor quality cable or a run
which is just too long. For high resolutions such as 1280x1024, the
maximum cable length may be as short as 25 feet or less for poor quality
cable. Better cable or fiber-optic repeaters may be necessary.

* Other similar problems - check cables for defective or improperly installed
connectors. This is especially applicable to cables with BNC or UHF type
connectors which require a kind of artistic talent to assembly properly and

If only 1 or 2 colors (of the R, G, and B) are effected, then look for
improper switch settings or bad connections (bad cable connectors are really
common) on the problem color cables.

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