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Test and measurement

An operator uses Tektronix’s WFM700 multistandard waveform monitor. The monitor features an HD/SD eye pattern display, one of the many different measurement displays engineers use to monitor digital signals.

Testing analog audio and video signals was predictable and understandable, once you learned what a waveform monitor and VU meter looked like. The test equipment showed representations of amplitude over time. Variants of analog signals and test equipment evolved over decades, but there was no radically new technology from the early 1950s to the 1980s.

Then along came digits, computers, compressed video, surround sound, complex RF signals like COFDM, and phase-shift keying (8-VSB, QPSK, 8PSK, 64QAM, etc.). The comfortable and familiar technology no longer sufficed at even the minimum level. A rich set of new tools for the new technologies has developed in the last 10 years. Vector modulation monitors, rasterizers, remote monitoring probes, surround sound scopes, MPEG syntax monitors and a host of other tools now allow us to look at complex, sometimes esoteric, signals. Some test instruments display only textual information, a far cry from the intensely graphic interface of the waveform monitor.

Video was easy when the scope represented the same thing that was on the wire. There was no need to decode for display. A simple special-purpose oscilloscope was created that locked to the repetition of the line or vertical frequency of the picture.

Once created, the sensible display allowed operators to see any defect in the signal and the displayed picture. With digital video, SMPTE 259 resembles wideband noise on an oscilloscope, and that is no mistake. It is actually one of the pieces that makes it work. Find the clock, decode the digits, deserialize the signal, convert the signal from digital to analog, and then display. Verify the correct format, find the embedded data and audio signals, and display them. That is a lot more stuff to do than we did only a decade ago. And audio (PCM AES, or AC-3/Dolby E compressed) or compressed video (ASI) could be on the same coax. ASI looks similar to SMPTE 259. Just because it looks like noise doesn't mean it is SMPTE 259M. Audio is similar, though AES digital audio is not turned into a noiselike signal for transmission.

For the most part, scopes today have moved away from the cathode-ray display. Even if the form factor is similar, most waveform monitors use digital sampling and a raster display. With some rasterizers, it is possible to put the output on computer displays ranging in size from a few inches to several feet across. This is one example of the merging of conventional technology and computer technology. If the display can be a VGA screen (XGA, etc.), the test instrument can also use computer analysis techniques.

Today, engineers use many different measurement displays. An eye pattern is a specially triggered and displayed version of the time vs. amplitude display. The impression is that the eye is a repeating pattern, like sync in analog video. But, in reality, it is visible only by overlaying all of the transitions from which digits are decoded.

Another radically different class of displays uses text to view the syntax of digital signals. SDI, AES and compressed video lend themselves to this technique. It is possible to have an MPEG signal that is perfectly decodeable in all respects except that the syntax breaks a rule or two and leaves the signal useless in some or all decoders. MPEG signals use a complex series of symbols that are legal only if assembled in a specific syntax. But that type of test instrument can display complex and rich information in away that is easy to understand. It displays statistics about the signal and its content with format information that allows the user to troubleshoot difficult and obtuse problems in a complex domain. The ability to set alarms for out-of-limit or missing items allows the complexity to be invisible when all is well.

Figure 1. New scopes display the spatial distribution of the surround sound signal, along with the levels in each channel. These scopes can be programmed for different metering ballistics to match in-house and international standards. Surround sound MSD600C from DK-Technologies shown above. Click here to see an enlarged diagram.

What about audio? How hard can it be with just two wires? Well, what about compressed 5.1 audio? How about Dolby E? How do you test AES? Specialized instruments analyze the content, the bit stream and the acoustic content (surround sound image). (See Figure 1.) A VU meter is interesting, but what about apparent loudness, content of the metadata or formatting of the data?

In addition to instruments that test the signal itself, we must also address the complex RF transmission technologies that deliver the signal to the end user. The ATSC's 8-VSB signal, as well as similar transmission technologies used for cable (QAM) and DVB-T (COFDM), require a new variety of test instruments that display RF in a way that allows the user to determine if the modulation meets the standard. Without these tools, it would be impossible to tell if the transmitter and its modulator are functioning properly.

These sophisticated tools are but a subset of the full suite of tools available. Waveform monitors can combine measurement capability for SD and HD signals, embedded audio, and even embedded compressed AC-3 or Dolby E. Modern facilities must have IT test instruments such as Ethernet testers and digital circuit test sets for T1 and DS3 lines. One key to successful measurement in an environment this complex is training on the right tools, and on analyzing the signals.

John Luff is senior vice president of business development for AZCAR.

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