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KRAMER ELECTRONICS LTD.
Some common problems
Sync-originating problems very often disguise themselves so that the data seems faulty, and the sync section is
not suspected at all.
The standard video sync level in Composite, s-Video (Y/C) and Component video formats is approximately 0.3
Volts (negative). Most signal acceptors, monitors, VCRs, etc., are designed to accept this level without any
problem. The tolerance in most cases is quite large. Most monitors are able to lock on signals as low as 0.2
Volts or even 0.15 Volts.
This is the basic standard. However, a host of acceptors such as video grabbing cards and special monitors
require a sync level which is substantially higher: 1 Volt, 2 Volts, 4 Volts and so on. Those acceptors usually
have a special sync input socket as in the RGBS format. If those acceptors receive a signal with lower sync
levels than they require, - which leads one to believe that there is something wrong with the data.
In Composite, s-Video (Y/C) and Component video formats the image does not get distorted or vanish. When
the sync level goes below an acceptable standard level, the image gets distorted or starts rolling on the screen,
but doesn’t vanish.
A common problem in high generation copies is attenuation of the narrow signals surrounding and within the
vertical sync (the serration and the equalization pulses that are needed for proper image display on the screen.)
The most common effect when the vertical sync is damaged is either a jittery image on the screen or the
“flagging effect” where the upper part of the screen is skewed sidewise and looks like a flag.
Processors which insert signals into the vertical blanking area – such as time code, teletext and copy protection
schemes - may cause image instability, mainly due to indirect sync deterioration as a result of the AGC
(automatic gain control) of the acceptor being activated unnecessarily. The result is image instability, rolling
picture, “flagging” and a host of other negative effects.
The main cause of the above-mentioned problems is improper analog sync levels. In the logic-level sync world
this shouldn’t happen. At least in theory! However, other problems – level oriented - also exist in the logic-
level sync world.
Until recently, logic levels were defined as 0 volts for logic level “0” and 5 volts (nominal) for logic level “1”.
There was a certain tolerance about where level “1” starts and what level can be declared as “0”. In the last
years, as computers got faster and faster and hence hotter, it was decided to drop some logic supply voltages
down to 3.3 volts and even lower. When this happened, the “0” level remained 0 volts, but the “1” level
became 3.3 volts. To make the “0” and “1” levels cope with the real world, the tolerance definitions were
changed. Now, in some cases, one device can send a logic signal of “1”, using 3.3 Volts logic or lower, but the
receiving device interprets this level as “0”. When it comes to sync signals, this immediately causes all the
negative effects mentioned above.
Logic incompatibility problems are treated quite well by most manufacturers using logic-translators circuitry,
but another “mine-field” is being set. The computer graphics world is very linked nowadays to the analog
world. Many video productions are made, edited and stored in computers. Some are outputted from the
computer using special cards, some use the graphics cards themselves. The analog world needs to get, in most
cases, a composite sync signal of 0.3 Volts. If computer related syncs are transformed to video levels, the
following problem: assume that in order to convert a standard logic level sync (5 Volts) to analog video sync
levels, a voltage divider from 5 Volts to 0.3 Volts is needed, a 16-fold division. If a logic level of 3.3 volts is
used, running at a lower tolerance of 2.8 Volts – then the division will result in a sync signal level of 0.17 volts
(2.8 Volts divided in 16.6) which is lower than acceptable. There’s no need to describe what will happen to the
analog signal!
Another catch – what if the horizontal sync is negative and the vertical sync is positive and they are translated
into an analog signal?
