A test of a single-frequency repeater in Washington this summer and fall indicates that it may be what broadcasters need to fill in coverage gaps in their mobile DTV service when they roll it out next year. "I'm cautiously optimistic," says OMVC's Sterling Davis (left).
Mobile DTV Repeater Shows Promise
From May to October this year, broadcasters’ Open Mobile Video Coalition conducted a live trial of their mobile DTV service in Washington, primarily to gauge consumer interest in the service and to learn something about how to market it when the commercial service rolls out late next year.
But the five-and-a-half-month effort also served as a test bed for a single-frequency (SF) repeater, a technology that mobile DTV proponents hope will improve mobile coverage.
Data from the experiment is still being evaluated, but Sterling Davis, VP of technical operations at the Cox Media Group and chair of OMVC’s Technical Advisory Group, said preliminary results suggest that repeaters may be the answer to spotty reception.
“It can definitely help mobile,” he said. “I’m cautiously optimistic.”
Victor Tawil, SVP of the Association for Maximum Service Television, who managed the experiment, said it was conducted “to help us understand better how feasible [the SF repeater technology] is, what you need to do, what are the pros and cons, how to design SF networks.”
He deferred to Davis for any comment on the results, but said that he fully expects broadcasters to use repeaters in certain circumstances, even though it will “cost some money.”
The Washington trial involved nine stations broadcasting 23 different mobile programs, including simulcasts of the regular broadcast services of the Fox and NBC O&Os (WTTG and WRC) and several cable networks, including Fox News, MSNBC, CNBC, MTV and Comedy Central.
For the repeater experiment, OMVC chose WNVC, a full-power station operating on ch. 24 under a construction permit from a tower in suburban Virginia, about 14 kilometers west of downtown Washington.
The station is owned by noncommercial Commonwealth Public Broadcasting Corp., which does business as MHz Networks. It and sister station, WNVT (ch. 30), offer a variety of foreign news services.
In an effort to improve reception of WNVC’s service in downtown Washington, particularly indoors, the OMVC experimenters built a low-power repeater operating on the same channel at WRC, the NBC O&O in Northwest Washington.
According to Tawil, Harris and SPX Communications Technology (formerly Dielectric) supplied the transmitter and antenna, respectively. Richland Towers, which owns the tower on the site and leases space to WRC, provided space for the repeater antenna. The Communications Research Centre Canada loaned test-and-measurement gear.
Although the Harris transmitter operated at just one kilowatt, mounting the antenna 153 meters off the ground boosted the ERP to 4 Kw, Tawil said.
Intel and Dell were deeply involved in the test, but they are more interested in mobile DTV reception indoors on portable netbooks than they are on walk-around smart phones and tablets, Tawil said.
In general, coverage of the mobile DTV signals during the trial was “pretty good,” Davis said. But it was not blanket coverage, he said. “You do lose certain stations in certain places just as you do with the fixed service. That’s not unusual.”
Davis said that broadcasters have high hopes for repeaters as “gap fillers” that will minimize lost signals. They inject stronger signals in “geographically challenged” areas where main signals are blocked by the terrain or buildings or other man-made structures.
The single-frequency repeater uses the same channel as the main station and its signal is synchronized as closely as possible with the main channel. Reception is made possible by the ability of digital TV receivers to reject multipath interference caused by signals on the same frequency hitting receivers at slightly different times.
In analog broadcasting, multipath caused by echoes of signals bouncing off buildings caused ghosting in the pictures. In digital broadcasting, such interference can cause pictures to freeze, pixelate or disappear entirely.
One alternative to SF repeaters are TV translators that operate on different channels. But there are drawbacks, Davis said. First, because they are on different channels, the receiving devices would have to retune to the new channel and that could take up to 10 seconds, he said. “That’s a more awkward solution from a consumer point of view.”
Second, he said, licenses for translators are not always readily available. The mobile provider would have to buy them, lease them or apply for new ones at the FCC.
And applying for a translator may be tough, involving finding a frequency and making sure it doesn’t infringe on anybody’s else’s signal, he said.
By contrast, he said, “You can get a license for [a SFN repeater] easily because you are interfering with yourself.”