When engineers first designed a digital TV station for noncommercial WPSX-TV Clearfield, Pa., they found that the signal would fall short of some key towns in its Johnstown-Altoona market.
Their solution: Instead of building one transmission site, they would build four—one primary and three carefully located facilities—all operating on the same frequency (ch. 15) in such a way as not to interfere with each other. Working together, the stations would blanket the sprawling, mountainous market.
The so-called single-frequency network (SFN) will be a first of its kind in the U.S., says George Thurman, director of operations at Penn State Public Broadcasting, licensee of WPSX-TV.
The SFN technology is the work of the Merrill Weiss Group, a consulting engineering firm in South Plainfield, N.J. Principal Merrill Weiss, a longtime proponent of SFN, has submitted the concept to the Advanced Television System Committee, which is looking for ways to improve coverage of digital TV stations.
DTV has been hampered by the industry-selected and FCC-endorsed transmission standard, 8-VSB. The standard has trouble delivering good signals to DTV sets, especially those with only indoor antennas. Also, the standard seems inadequate for broadcasting to mobile receivers.
According to Weiss, the key to SFN, or "distributed transmission," is synchronization of the multiple signals through various modulation techniques to minimize the echoes and ghosts that can disrupt reception.
Weiss says other broadcasters have expressed interest in building SFN systems.
As now envisioned, the WPSX-TV network will comprise a primary transmitter linked to an antenna on a 1,000-foot tower in Clearfield. Each of the secondary transmitters will make do with 200-foot towers.
Thurman says it will cost about $300,000 to build each transmission site. That's aside from the costs of studio-to-transmitter fiber and microwave, which must be installed for each transmission site.
The station hopes to bring the first secondary transmitter on line next month. If it proves the SFN concept and if the FCC approves, Thurman says, the other two secondary transmitters will be built in a year.
Right now, he says, WPSX-TV has FCC permission only for the first secondary transmitter.
Although 85% of the homes in the market have cable, Thurman says, cable operators have no obligation and currently no intention to carry the WPSX-TV digital service. "We have no choice. We find a way to get to them, or we have no [digital] viewers."
WPSX-TV viewers will be able to get good reception by aiming their antennas at the closest site in the chain. And the station will be able to modify the coverage by adjusting power at any of the transmitters. The system permits use of full power at any or all of the sites.
And because sites are staying on one channel, they should not interfere with other broadcasters, says Ed Williams, a senior engineer with PBS, who is not associated with the project.
There is precedent for the SFN technology. Cellular phones and paging services use multiple low-power transmitters to cover entire cities. In areas of the country where difficult terrain makes coverage from a single site difficult, broadcasters have for a long time used translators—low-power transmitters—to rebroadcast the primary signal and fill in holes in coverage. But the translators typically work on different channels.
In WPSX-TV's case, Thurman and his engineering team considered using translators but were not able to locate enough unused channels.
SFN technology is not without problems, however. Williams says most of the existing equipment necessary for SFNs was designed to work with the COFDM transmission scheme, not the 8-VSB standard. Before SFN could be widely implemented, manufacturers would have to develop and test new product.
It's also more expensive to build and operate an SFN, which requires not only multiple transmitters and synchronization equipment but also the links to tie them all together, Williams says.
Art Allison, director of advanced engineering at the National Association of Broadcasters, says it is unclear whether SFN technology could be deployed on a large scale, given the "very narrow and limited frequency band that we have."