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DTV goes to the country

TV engineer successfully tests 8-VSB over challenging terrain 11/01/2008 08:00:00 PM Eastern

Many in the industry have predicted that the digital television system as approved by the ATSC, which is problematic even in some urban areas, would never work in the nation's rural communities serviced mostly by some 6,000 translators. Recent tests in rural Utah prove otherwise.

It has been assumed that, because of long distances from the originating station and uneven terrain that inhibits line-of-sight to the transmitter, over-the-air DTV using 8-VSB modulation was unlikely to reach the consumer's home with a standard RF antenna.

After four months of continuous testing with two long-range signal repeaters, Kent Parsons, an engineer for the University of Utah's two educational channels, vice president of the National Translators Association (NTA) and a translator guru for nearly 45 years, has reached the conclusion that DTV translators will be able to deliver "studio-quality" television signals to rural viewers with high reliability and reasonable cost.

Looking to find out just how translators would work in the demanding environment of Utah—a state with approximately 300,000 viewers who depend on translators for TV service—Parsons found that digital translators can send a reliable signal, using modest power, and will eventually cost about the same as analog translators. He plans to continue his tests well into the summer months—when lightning can be a problem—and then send a full report to the FCC.

In December, armed with two-year experimental licenses from the FCC, Parsons began to measure the effects of DTV on translators after no one in the industry seemed to pay attention to the fact that the university's stations could possibly be taken off the air if interference problems should arise. With no Class A status (granted by the FCC to all full-power stations in the U.S.) and no channel assignment to ensure their survival, he took action.

"Translators have been completely ignored by Congress, with no assignments or consideration of being displaced," he says. "I believe that, because of the debate surrounding the modulation scheme, some felt that the last thing this industry needed was another set of test results that would muddy the water. I kept waiting for someone—the Congress, the FCC or even the NTA—to do something to make provisions for translators to continue to provide free over-the-air TV to rural people."

Judging from his findings, Parsons is sure that stations will be able to produce quality service for the hundreds of thousands of rural TV viewers and the many cable systems that rely on translators.

Because it wasn't line-of-sight, many people surmised that the digital signal would not get through. Preliminary indications from Parsons' spectrum analyzer, however, showed that it was going to work. So, last fall, he purchased an RCA DCT100 digital set-top receiver and hooked it up to a receive antenna. The signal was crystal clear and has been ever since, through fog, snow storms, rain and other natural challenges.

He next went to the FCC and requested experimental authority for two locations to stage his own reception test. Engineers there were very helpful, Parsons says, and assigned him call letters—although they made him sign an agreement to use only 8-VSB modulation in his tests. He also received permission from KSL-DT, the NBC affiliate in Salt Lake City, to use its signal for his tests.

KSL-DT, on ch. 38, is received at a mountain-top receiver site at 8,500 feet above main sea level (AMSL), 83 miles from Salt Lake City. A Zenith 8-VSB "remodulator" upconverts the signal to ch. 17 with a 30-W translator for local distribution and relays it to the next site. Parsons chose ch. 17 because an existing analog station operating on ch. 16 is also inbound at the site and he wanted to see if it would interfere.

"I wanted to create a worst-case scenario," he says. "We had no problem with the adjacent ch. 16. We also found no problems with ambient floor noise at the translator."

At the second relay site, 67 miles from the first site (on Cove Mountain, 3 miles east of Monroe, Utah, at 8,750 feet AMSL), ch. 17 is converted, again by the Zenith remodulator, to ch. 32. This signal is then fed into two power amplifiers in a 1980s Larcan/TTC XL20 TV translator with an output power of 1 W for distribution to Monroe. Parsons has been watching DTV in his home off this translator and is encouraged by the results. There is, however, a 11/2-second delay in the video signal.

Parsons and his two sons use a standard TV antenna at their respective homes, and all are receiving a reliable and sustained picture.

"We decided that we wouldn't do anything different for DTV than we already do for NTSC," he reports. "Actually, these tests have also proven to me that people in rural areas will have a smoother transition to DTV because they already have the antennas up on their roof and don't care what channel the signal is on. So we're in business."

Parsons says the tests show that the new Zenith exciters will work with older amplifiers if aligned properly, so existing translators can be retrofitted. He also says he's disproving the myth that more translators will be needed for DTV.

"I believe we will be able to use translators not only for distribution but also for relay links in place of microwave. We can go long hops with very little power," he says. "I also believe that we can put DTV signals in between analog signals that we're transmitting and transport a second channel. This is the first time that we've been able to take signals and improve them."

Parsons believes that second-generation DTV receivers with 8-VSB decoders are working a lot better than first-generation units. "We now know that [signal] paths do not absolutely have to be line-of-sight, and we know that the technology is available," he adds. "We're ready."

 

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