A number of display systems primarily used in specialized areas such as aviation, space (NASA), and military/defense employ non-standard video signal technologies such as raster/stroke analog video. Displays in cockpits of planes, pilot helmets, and ground vehicle systems, for example, employ some of these non-standard video techniques. Testing these displays requires multiple specialized video signal generators and pattern generators.

In the past, these systems have mainly been built with proprietary systems employing a large amount of custom software and hardware and long development times. With arbitrary waveform generators (arbs) and the PXI platform, customized raster/stroke video signals can be created in a cost-effective manner. Some key differentiators of the arb critical for the creation of multiple signals that comprise raster/stroke video include auxiliary digital pattern outputs and deep on-board waveform memory. With its timing and synchronization features, PXI provides the platform needed for multiple arb synchronization.

With arbs and software, you can create custom video patterns of virtually any format. Utilizing the synchronization capability of arbs and the PXI platform triggering buses, multiple PXI arbs can be integrated to output the signals necessary for raster, stroke, and/or raster/stroke video.

The first step is to define how many signals are needed to operate the display technology in question. Of the signals needed, you need to determine what are custom, standard, and digital synchronization signals. Raster/stroke video would require the maximum number of signals. The raster video signal is typically a standard NTSC/PAL/SECAM video pattern, and the stroke video overlaid on top of the raster video pattern is a customized signal. This can require up to eight signals. Stroke video can require three color mode control signals, X and Y deflection signals, and the bright up signal (a total of 6). Raster would require one composite video. An additional raster/stroke signal is needed for determining what mode the electron guns are placed in.

Once you have determined the number of signals needed to drive your display, you need to determine the number of arbs needed to produce all signals. A standard video generator or an arb can be used to generate standard video patterns along with custom ones. Typically, one associates a single-channel analog source with one output, but with arbs you can derive multiple signals from one arb due to the digital pattern generation feature, thus reducing the need to have one signal source per signal.

Figure 1: Example of a Display where both Raster/Stroke video is employed. The box and concentric circles are overlaid in stroke mode over the background NTSC raster video gray scale bars.

Arbs are adept at generating 16-bit high-speed digital output patterns in addition to the main analog output channel. The 16-bit digital representation of the analog waveform is available as digital pattern outputs along with the update sampling clock signal to which it is synchronized. You can take advantage of this in two ways: using the arb as an all-digital pattern generator or as an analog video generator with four custom digital output signals.

The multiple signals need to be tightly synchronized to ensure that the image reproduced on the display is reproduced accurately. The loss of synchronization essentially leads to blurred or choppy images, so the timing of the multiple arbs needs to be synchronized with high accuracy. The issue boils down to accurate clock synchronization and simultaneous triggering of the arbs.

When generating multiple video signals, you must synchronize the sample clocks of your arbitrary waveform generators together. There are two methods to synchronize your arbs output clocks: Phase Lock Loop (PLL) and external clocking.

The PXI platform can be used to synchronize multiple arbs with the following timing and triggering features:

  • System Reference Clock — The 10 MHz clock is independently distributed to each PXI peripheral slot through equal-length traces with a skew of less than 1 ns between slots. Multiple devices can use this common time-base for synchronization. This allows each arb to phase lock to the system clock.
  • PXI Trigger Bus — This bus features eight bi-directional lines that link all PXI slots, providing inter-device synchronization and communication. The skew from slot to slot is less than 10 ns. A "start" trigger signal can be distributed through one of the PXI trigger lines from a "master" arb to all slaved arbs to initiate waveform generation simultaneously on all arbs in the scheme.

If you need to synchronize to an external reference clock source such as a rubidium source rather than the PXI 10-MHz reference clock, you can connect the common reference clock to all the PLL reference connectors on the front panel. A 10 MHz reference clock signal is recommended as most instrumentation can phase lock to 10 MHz reference clocks.

Figure 2: The Raster/Stroke Signals that reflect the image in Figure 1.

Another method of synchronizing multiple arbs is direct external clocking. If the arbs are capable of accepting direct external clocks in lieu of its internal clocks, then it allows for another method of synchronization. This flexibility is useful for applications that require a precise clock source, which is not possible using the default counter-based clocking scheme.

To output a signal requires you to download the signal data to the arb internal memory. This is a large amount of data that must reside inside the arb for it to continually output waveform data.

The construction of the video test waveforms can be involved and demanding. One direct approach is acquiring the analog video and digital sync signals from a working device under test (DUT) with a high-speed digitizer filtering, processing, and playing them back out with the arb. It is important to phase lock loop or externally clock the digitizer to the arb's timebase to ensure that the timing is accurate. Digitizers sampling at 100 MSamples/sec are ideal for the task.

Another useful feature of arbs is that they can programmatically adjust the signal level and offset while outputting waveforms. This does not affect the output resolution or the shape of the waveform, just the amplitude and DC offset. It's very useful for testing how the display device tolerates thresholds for syncs and signal level and can also be used to tailor the system to different impedances and line lengths.

PXI arbitrary waveform generators have a flexible set of features including digital pattern generation, deep waveform memory, and synchronization that allow them to be utilized as custom analog and digital video generators.

This work was done by Dr. Lokesh Duraiappah and Jerry Lopato of National Instruments . For more information, contact Dr. Duraiappah at 512-683-5601; e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it..

NASA Tech Briefs Magazine

This article first appeared in the May, 2003 issue of NASA Tech Briefs Magazine.

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