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Tri-Level Sync in a Bi-Level World


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Tri-Level Sync in a Bi-Level World

by Dave Pincek, Vice President of Product Development


The 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.


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.


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.




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 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.



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