Specifications

April – June 2002 ExtroNews 13.2 17
TECH CORNER
synchronization in the new television
system. Tri-level sync eliminates the DC
component and provides a more robust
way to identify the coming of
synchronization in the signal chain.
Tri-Level Sync
Tri-level sync was introduced with the
SMPTE 240 analog HDTV standard.
Previous to that, the early HDTV 1125/60
systems used various synchronization
waveforms, as provided by various
1125/60 equipment manufacturers. The
creators of the later SMPTE 240 HDTV
standard searched for a standard sync
waveform that would ensure system
compatibility. The goal was to provide
more precise synchronization and relative
timing of the three component video
signals. HDTV component video has sync
present on all three channels: Y, Pb, and Pr.
In addition, the sync structure needs to
be resilient enough to endure
multigenerational recording and other
noisy situations. Tri-level sync met the
requirements.
Figure 3 shows a graphic representation
of a tri-level sync signal. As defined by the
SMPTE 240 standard, the pulse will start at
the zero volts (specified black level) and
first transitions negative, to -300 mV (+/- 6
mV). After a specified period, it transitions
positive + 300 mV (+/- 6 mV), holds for a
specified period and then returns to zero
or black level. The display system “looks”
for the zero crossing of the sync pulse.
Each half of the tri-level sync pulse is
defined to be 44 samples (reference clock
periods) wide, for a total sync pulse width
of 88 samples. The rise time is defined to
be four samples wide +/- 1.5 samples.
This symmetry of design results in a net
DC value of zero volts. This is one major
advantage of tri-level sync. This solves the
problem of a bi-level signal introducing a
DC component into the video signal. The
elimination of DC offset makes signal
processing easier. Within our new digital
television system, the unique excursions of
the sync derive numerical values that are
easily coded and easily recognized within
the digital transmission channel.
Converting Tri-Level to Bi-Level Sync
There are times when it is necessary to
convert tri-level sync to bi-level sync such
as when component HDTV is converted to
RGBHV. A format converter, like Extron’s
CVC 200 will perform the conversion of
tri-level to bi-level sync as part of the
component HDTV to RGB conversion
process. Traditional displays and projectors
not capable of handling tri-level sync
will “see” sync information in the
traditional way.
Any time signals are converted from one
format to another; the relative timing of
the conversion is of prime importance. The
introduction of timing error, once
introduced into a signal channel, is difficult
to repair. The positioning of tri-level sync
with respect to active video and the wider
excursion from peak negative (-300 mV) to
peak positive (+300 mV) provided by this
format establishes easier sync detection
and more consistent triggering through
the use of the zero crossing. When
converting bi-level sync, the leading edge
of the bi-level pulse should be aligned
using the zero crossing of the tri-level sync.
By doing so, the bi-level sync pulse will
provide leading-edge trigger at the proper
point and correct timing will be
maintained. Figure 4 shows the
relationship of a tri-level sync signal to a
properly-timed bi-level sync signal.
Anyone involved in interfacing video
signals will, at some point, encounter the
need to convert tri-level sync to bi-level
sync. As time progresses, a growing group
of displays and projectors will be designed
to cope directly with these format
differences. In the meantime, technicians
should be aware of the differences in sync
construction and the proper timing
relationship for conversion between these
two common formats.
Figure 4
Figure 3