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KRAMER ELECTRONICS LTD.
The vertical sync, sometimes referred to as the field sync, is also a negative set of pulses at the beginning of
each video field (60 field/sec. in NTSC, 50 fields/sec. in PAL). The vertical sync is a more complicated signal
than the horizontal sync and it tells the monitor – “be prepared, a new field of data is coming”. The vertical
sync also resides in the blanking area, and is not seen on the screen.
SYNC Location
Sync signals appear in the following locations in the various video formats:
In the Composite video signal, sync is an integral part of the signal as the Composite video signal is a package
including horizontal and vertical sync, luminance and chrominance (color) information.
The s-Video signal format is made of two parts – the luminance (brightness, black & white information) and the
chrominance (color information). The sync signals in this format are part of the luminance (Y) signal, in a
manner similar to that of the Composite video signal.
In the Component video signal, which is comprised of three signals – the luminance signal (Y) and the two color
difference signals, R-Y and B-Y - the sync signals reside in the Y signal, as in the Composite and s-Video
signals.
In RGB (Red, Green, Blue) signals, used in professional computer graphics and display applications, there are
several options for the sync signals to be carried along with the data.
The most common is called RGBS. It has a separate Analog Sync channel – and there are therefore 4
wires carrying signals, Red, Green, Blue and Sync.
Another popular format uses only three wires – Red, Blue (as in the above configuration) and
Green+Sync, where the sync rides on the green signal. In this format, the signal levels are not
identical, as the Green signal has a larger amplitude due to the sync it carries, (and the sync signals
must be stripped off in order to recover the normal green sync level.)
A subset of this format inserts the sync signals in all three data channels – e.g., Red+Sync,
Blue+Sync and Green+Sync.
The format generated by a Computer Graphics card is normally made of 5 different signals: Red, Green, Blue,
Horizontal sync and Vertical sync (sometimes referred to as Hs and Vs.) To make things more complicated – the
data channels – R, G, B - are analog (their level varies in a continuous way from 0 to maximum) but the sync
signals are digital – (TTL level) being either 0 or 1 (0 volts or 5 volts). To make things even more complicated, in
contrast to the analog sync signals which are negative pulses, the computer generated logic-level syncs can
appear in both directions. Sometimes one of the syncs is positive and the other is negative, sometimes both are
either positive or negative. The sync direction is dictated by the source, the graphics card, and is dependent on the
resolution required. In the past, the sync direction instructed the monitor which resolution to choose. Nowadays,
most of the monitors are smarter and set the resolution automatically, but nevertheless, the cards still generate
those sync signals.
Another twist to this format is the fact that in most cases the logic-level syncs are separated and each
sync runs in its own channel, but there are cases when the logic-level syncs are composite – joined
together in the same mix of directions.
In the digital world – SDI, DV, MPEG and other formats, the synchronization signals either travel in a special
digital sync channel or, in most cases, are embedded in the digital signal. Retrieving the sync information when
embedded in the digital signal involves complicated circuitry and the problems related to digital syncs will not be
discussed here.
