5
KRAMER ELECTRONICS LTD.
Solutions
The first step in choosing a solution is to correctly identify the problem. When data disappears from the screen,
the initial response is to look for problems in the data path, but it should be born in mind that it could be a sync-
related problem. A technical person, using an oscilloscope, can monitor the data channels and the sync channels
and the answer is straightforward. If the problem identified is sync related:
When analog sync levels are too low, a standard video amplifier with level adjustments should be used. The sync
is inserted as a video signal, and by using the controls of the amplifier, the level is adjusted to the standard level.
When an analog sync signal seems to be distorted at the vertical blanking area – missing equalizing pulses or
serrations or having sync disturbing information in this area –an expensive solution, such as a TBC (Time Base
Corrector) can be used, or less expensive, dedicated machines may be used. Such machines include sync
stabilizers which “clean-up” the teletext, time-code, closed-caption and other information and machines such as
Black-Burst Restoring devices which replace the whole horizontal and vertical blanking area, and the color burst,
with newly generated signals.
When logic level syncs are involved, it should be decided whether the sync direction is the problem – (positive
instead of negative, for example) or the analog-converted sync level is wrong. If the problem results from wrong
sync direction, then devices that “rectify” the syncs into the right directions are needed. Sync-direction-rectifying
processors are usually part of other devices which combine TTL syncs with analog signals (Sync-to-Green
Adders, for example). Those devices usually include additional circuitry that may solve the second problem –
syncs with wrong logic level. Those devices include a sync logic-level translator to correct level incompatibility.
Even when everything seems correct – the sync source generates a signal within its specifications and the
acceptor is designed to accept standard sync information - a quick check to determine whether both machines
“talk” the same language is needed. Please refer to the last page of the manual, where the technical specifications
are described.
If a sync signal rides on all the data signals – Red, Green and Blue - and the signals are inputted to a machine
which accepts Red, Blue and Green+Sync, the wrong color will shown on the output, usually with a purplish tint.
This is the result of feeding Red and Blue signals higher than normal (as they “ride” on the sync). To solve this
problem, some acceptors have built-in switches or software commands to ignore those incorrect signals and those
controls should be activated. Alternatively, a “sync-shaver” machine should be used, one for each of the Red and
Blue channels. This device “shaves-off” and strips the sync signal from the data signal. This is a very common
problem and the above-mentioned simple solution is often ignored.
Sometimes, a video image shows instability due to slowly rolling horizontal bars. It can be assumed that this is a
sync-related problem, but very often it’s not. Image instability and rolling bars can result from ground-related
problems. When two devices are connected with a cable, they may be fed from sources with different ground
potentials. The difference in ground potentials (which should be “0” volts) creates a ground induced current that
flows with the signal, modulates it and creates the phenomenon described above. The solution to this problem is
ground isolation. It can be done either by special transformers (that unfortunately might impair the frequency
response of the signals involved) or by electronic devices utilizing opto-isolator technology or other electronic
means. Checking whether this is the source of the problem is easy – using a voltmeter simply measure whether a
ground potential exits between the two points. Care should be taken to avoid touching devices or wires carrying
dangerous mains voltages.
Genlock
Video signals coming from different sources have different timing (time based sync relationships) and different
random color phase relationships. When only one video source is used, there is no problem. In professional
applications all the video sources should be synchronized. Synchronization involves both the syncs and the
color phase.
Synchronization is needed mainly in two applications – video mixing and video switching / routing. A video
mixer or SEG (special effects generator) takes two sources or more and blends them together. This process
creates nice looking transitions from one scene to another, and enables many artistic special effects. The video
sources may not be mixed or blended unless they are synchronized one to the other or “Genlocked”. One of the
sources for the term Genlock is the abbreviation of the words GENeral LOCKing, which means that the sources
are “locked” to each other sync-wise and color subcarrier-wise.
The other important application requiring synchronized (genlocked) sources is video switching and routing. If
two sources are used, and the operator switches from one source to the other, if the sources are genlocked, and
the switching occurs during the vertical interval blanking period, a clean transition occurs, free of artifacts and
image breakdown. The receiving end of the switched sources does not need to resynchronize its circuitry to the
newly switched source, resynchronization does not take place and the transition is smooth.
