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DTV Expands Reach

DTV signals just found another route into homes. The ATSC's (Advanced Television Systems Committee) recent recommendations give broadcasters greater flexibility in reaching viewers.

Until now, broadcasters have had only one approach to DTV transmission: a tall tower with a high-power transmitter blasting the signal across the market. The new approach, called a Single Frequency Network (SFN), allows the use of smaller towers with lower-power transmitters. SFN requires all transmitters to emit the same signal and receivers to accept signals from more than one transmitter without interference.

Here's the bonus: Using a smaller transmitter ultimately enables a broadcaster to reach a greater service area. Synchronized multiple transmitters can best serve a station's market by extending the signal range, increasing signal levels or filling in gaps caused by mountains, valleys or tall buildings.

"Broadcasters can get more-uniform coverage for indoor antennas, while reducing interference to neighboring stations," says Merrill Weiss, developer of the ATSC technology for transmitter synchronization and a champion of SFN as a broadcasting tool. Nat Ostroff, who sits on both sides of the industry as Sinclair's vice president, new technology, and president of transmitter maker Acrodyne Industries (Ai), says the development will reinvigorate the low-power–TV industry.

The SFN approach has been in service for over a year at WPSX(DT) Clearfield, Pa., a station that serves as a test bed for the development of single-frequency networks and transmitter-synchronization techniques. At WPSX, additional transmitters were used to overcome the mountainous terrain of central Pennsylvania.

"Hardcore application of this approach may be in mountainous states where TV translators will be replaced with an SFN," says Ostroff.

But the trick to SFN is uniformity.

It's critical to have transmitters emit the same output signals on the same frequency: Any difference and the receiver will interpret the signals as separate signals, not echoes. The result? Interference and the inability to receive the signal properly in areas of signal overlap.

A second concern is financial. Is one transmitter or many a better economic proposition? It depends on the individual station's power bills, maintenance costs and land-access fees.

Still, Weiss says SFN could have long-reaching appeal.

He believes a jointly developed, co-

located SFN could be a boon to broadcasters involved in initiatives like USDTV or the over-the-air multichannel cooperatives proposed by the Emmis Station Group. Stations could gain economies of scale by building their networks together, sharing the maintenance and operational costs, and ensuring that as many viewers as possible receive all signals.

But challenges remain, such as coping with a subscriber who doesn't receive signals from some stations and, therefore, cannot get the entire service package. An SFN approach could obviate outdoor antennas, making it far more attractive to consumers.

"You don't want to have people clambering on subscribers' roofs to install a service like that," says Weiss. "You want a subscriber to be able to take home a set-top box, add an indoor antenna and get the signals."

Weiss says the new approach could be attractive to stations that want to avoid retrofitting a small transmitter and tower with a big antenna and a large transmitter. Three or four medium-power transmitters or three or four dozen low-power transmitters scattered throughout the market in a cellular approach could be more effective in providing coverage than a behemoth, megawatt-class transmitter.

Ostroff disagrees. He considers the multi-transmitter approach an unattractive station option. "It's a lot easier to deal with a single transmitter and tower site than a dozen or more satellite cellular-type installations," he says. "I don't see broadcasters being comfortable with that type of arrangement."

Yet he does see it as an effective way to reach viewers on the fringe of a market.

"Fifth-generation receivers from Zenith allow for indoor reception, but the signal will need to be strong enough to reach indoors," he says. "Using an SFN so the signal [on the outskirts of a market] is the same as it is [near the main transmitter] would be very interesting."