Specifications

14 ExtroNews 14.1 January – March 2003
Eliminate Your Sync Problems, Part II
A Shotgun Approach to Troubleshooting Sync Problems in RGBHV Systems
O
ne of the most common problems affecting high-resolution display systems is bad sync, which occurs
when either one (or both) of the H and V signals is not effectively delivered to a display device.
As discussed in the last issue of ExtroNews
bad sync can exhibit itself in a number of
different ways, including a flashing image or
no image at all. Bad sync can also be the
result of reflections and cable problems.
Troubleshooting these scenarios will be
addressed in this installment.
Scenario 3 – Sync Signal Reflections
Reflections present in H and/or V sync
lines may result in flashing, rolling,
scrambled images, or no image at all. Sync
reflections occur when the output
impedance of the H and/or V sync lines from
a source, such as a computer graphics card,
are not matched to the input impedance of
a display device, such as a projector. While
the industry standard for the input and
output impedance of RGB connections is set
at 75 ohms and well adhered to by
manufacturers, no adherence to such a
standard exists for sync connections.
Because this standard is not observed,
reflections are present in almost every sync
line in existence. Reflections are exacerbated
by longer cable runs, higher sync
frequencies, faster rise times, and disparity
in the characteristic impedance of cables
with respect to the input and output
impedances of sources and displays. As a
result, sync reflections can cause problems,
but don’t always, and can be very hard to
actually verify. Fortunately, they are usually
easy to fix.
Solutions:
Resolutions to sync reflection
problems depend on the system in which
the problem exhibits itself.
A) A computer cabled to a display device
directly, typically through a longer run of
cable without a computer-video interface or
distribution amplifier (DA) to buffer the sync
lines, is the most common scenario in which
sync reflection problems will exhibit
themselves. Try buffering the sync signal at
the source by installing a computer-video
interface, DA, or a line driver, such as an
Extron Peaker or Extender, at the computer
to resolve the problem.
B) A display device cabled to a source in a
system that includes computer-video interfaces
or other sync buffering devices can also
occasionally exhibit sync reflection problems.
This may occur with some specific sources but
not others. Try matching, or more closely
matching, the sync impedance by adding
additional termination at the display device, as
follows:
On display devices which use BNC
connections, remove the sync line at the
display device and attach a BNC T connector
with a 75 ohm terminator to the sync input
of the display device. Then, connect the sync
line to the remaining connector on the BNC
T connector. Try the V sync line first, then the
H sync line, then both. This must be done at
the input to the display device, as opposed to
attaching such “double termination”
upstream at the output of the signal source.
Attaching this kind of “double termination”
at the output of an upstream device will
increase an impedance mismatch, and
probably make this problem worse.
On display devices which use 15-pin HD
connectors, an Extron Laptop Sync
Termination Adapter (LSTA) or Laptop Sync
Coupling Adapter (LSCA) can be used.
Remove the input cable from the display
device’s input connector, plug the LSTA or
LSCA into the vacated input, and plug the
cable into the input of the LSTA or LSCA.
Again, these devices must be used at the
input of the display device for the same
reasons noted above.
In more severe cases, an active or powered
termination must be used at a display device.
An Extron Active Sync Termination Adapter
(ASTA) can be installed at display inputs with
a 15-pin HD input, or an Extron SS 200 can
be used at display inputs with either a 15-pin
HD or RGBHV connectors. These are the
most expensive pieces of corrective
TROUBLESHOOTING TIPS
A BNC T connector is used at the display device
input for additional termination and to reduce sync
reflection.
An Extron Laptop
Sync Termination
Adapter (LSTA) is
used for display
devices that have a
15-pin HD connector.