Specifications

16 ExtroNews 13.2 April – June 2002
Residual sync results from incomplete
removal of the sync information from a
video processing channel. Sync is typically
imposed on the green channel in RGsB
systems. High definition component video
signals contain sync on each channel.
Depending on the performance
characteristics of the DC restoration
circuitry within the video processing
channel, some or all of the sync pulse may
not be removed from the green channel.
Residual sync causes the green channel to
bias incorrectly with respect to red and
blue at the display CRT, thus causing a
color shift. Even in RGB systems where
sync is introduced on all three channels,
there is some difficulty with maintaining
consistent processing between the three
channels. Again, small DC shifts in the
black level caused by residual sync can
disturb the color balance or gains of the
video channels.
A significant amount of power is used
by the broadcast transmitter to send the
sync pulse. Polarity of the video signal is
designed to minimize the amount of
power used to transmit sync. And, while
we have not transmitted analog versions
of high definition television terrestrially,
early testing done during HDTV
development demonstrated a need
to improve the management of
Looking for the sync pulse has always
been one of the “trickiest” of tasks for the
display signal processor. It requires careful
biasing of the sync processing circuitry so
that the sync pulse is made as
distinguishable as possible from the other
voltage levels within the video signal. As
part of the video signal, bi-level sync
introduces an unwanted DC component
(Figure 2). In processing of composite,
S-video, or component video the DC
component is not too troublesome and
can easily be managed as part of the
normal sync separation routine. When
bi-level sync is introduced onto RGB video
channels, the process is more complex. In
some systems, sync is introduced on the
green channel only. This requires that the
sync separation process be ultra clean; in
most cases, however, it is not. Usually
a very narrow sync pulse remains.
Bi-Level Sync
Bi-level sync has been the standard
synchronization signaling method for all
forms of video including computer video,
composite video, S-video, and component
video. Bi-level refers to two levels. For
sync, this means a pulse having two
voltage levels (a high and low level,
relatively speaking), hence the name.
Systems using bi-level sync are edge
triggered. Typically, the negative-going,
leading edge of the pulse triggers the
synchronization process (Figure 1). Display
systems must “look” for this negative
going edge in order to identify the
moment in time when to re-sync the raster
scan process. Most will recall that
computer graphic cards sometimes output
positive-going sync. Positive-going sync
signal the display that the graphics line
rate has changed to a new format.
Tri-Level Sync in a Bi-Level World
T
he advent of HDTV has brought a number of new concepts and technologies with it. One of the concepts
put into practice is tri-level sync. Tri-level sync solves some traditional problems found with bi-level sync.
Although tri-level sync is preferable with the new television system, we still find ourselves interfacing to
systems capable of handling only bi-level sync. Therefore, the need exists to convert from tri-level to bi-level
sync on occasion. This Tech Corner will acquaint the reader with the new tri-level sync format and its
relationship to bi-level sync.
TECH CORNER
Figure 1
Figure 2