WASHINGTON Although the concept of Distributed Transmission Systems (DTS) has been around for a number of years, and has even been deployed experimentally, the FCC has been slow to issue its formal blessing. That finally changed this week when the commission issued an official green light to implement the technology that closes holes in a digital television station’s service area.
DTS—also referred to as single frequency networking or on-channel gap filling—originated some 17 years ago by S. Merrill Weiss, president of Merrill Weiss Group LLC, who said that while he’s pleased with the FCC decision, the champagne glasses are not coming out of the cupboard just yet.
“I’m not planning a celebration; there’s still a lot of work to be done,” Weiss said. “I think that it will be a slow rollout, but there are some stations that will benefit immediately. It will take the FCC some time before the rules are fully in operation.”
Richland Towers prepares to install an SFN antennain Times Square in New York Cityduring a DTS test in 2007.Weiss said he expected some of the first stations that might take advantage of the technology would be those with significant coverage issues, but it could eventually be used more widely by those wishing to engage in handheld and mobile TV broadcasting, as good signal coverage is especially important for those applications.
Jack Wilson, director of marketing and business development at Axcera, a company that has been heavily involved in DTS implementation and testing, shared these views.
“Field testing has shown that DTS can significantly improve coverage in a broadcaster’s service area, most notably filling in coverage gaps caused by terrain shielding or transmitting antenna deficiencies,” he said.
He further acknowledged that the technology would pay a big part in making small screen television a reality.
“With the progression of ASTC M/H technology, DTS will provide a significant coverage advantage for mobile TV services,” Wilson said. “A single high-power station isn’t going to provide the service required for mobile handheld devices with their lower gain receive antennas that are only four feet off the ground, then a single high-power station isn’t going to provide the service required. You will also need more low power sites for in-building coverage—this would be a driving factor for DTS deployment. I believe that the most ubiquitous adoption of DTS will be for mobile services to achieve the desired coverage, especially in-building penetration.”
DTS was made possible with the advent of digital television broadcasting. Multiple transmitters operating on the same frequency and sharing the same program feed can provide service to viewers’ homes and their mobile and handheld devices that otherwise might not receive adequate signals. With analog television, the multiple signals would hopelessly jam one another. However, digital demodulators can be designed to allow receivers to lock onto the most viable DTV signals and ignore the rest.
TALL TOWERS ON THE WAY OUT?
As DTS technology can provide coverage into areas totally unserved by conventional tall transmitting towers and high-power transmitters, Weiss was asked if he thought it might eventually spell the end of the tall tower era of broadcasting.
“No, certainly not in the near-term,” Weiss said. “There’s too much investment in tall towers right now. I see most stations using gap fullers where they have areas [to service] that are obstructed from tall towers, or where they have a lower power operation and can use the technology as a service extender. Either way, it keeps high-power tall tower operations going. In some situations, broadcasters could save money and provide more reliable service, but I think that this will take a long time.”
For stations wishing to implement DTS, at minimum, a new transmitter, antenna and methodology for obtaining a signal to transmit must be provided. Weiss estimates that putting a “gap filler” on the air could run between $50,000 and $150,000.
“It depends a lot on the size of the facility,” Weiss said. “Some of the cost will depend on how the facility gets the signal.”
Some DTS installations may be able to get by with off-air pickup of the broadcast signal from the station’s primary transmitter. Others may require fiber interconnection or the use of microwave radio facilities for receiving a signal for transmission. If the area to be covered is large and a station has a multiple secondary transmission sites in areas not easily served by fiber or microwave, then satellite distribution is a possibility.
THE WILMINGTON EFFECT
As part of the fallout from the September Wilmington, N.C. DTV trial, at least one station in that market—WECT—heard from longstanding viewers that they could no longer receive its programming after the analog transmitter was shut down. Weiss was asked if he thought that DTS technology might be the best way to cope with this and other post-analog coverage problems.
“What they [WECT] ended up with was a replication gap,” Weiss said. “The problem is that this particular station—even if they had not moved their transmitter closer to the ocean to better center it in their real market—would not have been able to replicate their analog coverage, as they were moving from low VHF to UHF.”
Weiss said that it was too early to know if the commission would allow such stations that will no longer be able to cover their original service area to expand their reach with DTS.
“I can’t answer that at this point,” he said. “We probably won’t know until the full Report and Order is released.”
Wilson said that single frequency networks make more sense than going with traditional translator technology for filling such coverage gaps.
“Certainly DTS is the most spectrally efficient solution when broadcasters complete the build out of their coverage area,” he said. “DTS is operationally a better solution than rebroadcasting on different channels.”
MORE WHITE SPACE ISSUES
The commission’s decision came on the same day it voted to permit unlicensed “white space” devices to operate within spectrum previously set aside exclusively for television broadcasting.
A sizable group of white space device proponents view DTS as a direct threat to maximizing opportunities for white space operation—more TV transmitters in more places equates to less unused spectrum.
In comments representing the views of more than 20 DTS opponents filed with the commission more than two years ago, the New America Foundation and the Media Access Project declared that a favorable decision on DTS would provide “a redefinition of broadcaster’s service areas to include many of the white spaces not currently covered by a broadcaster’s current single high-powered transmitter,” pointing out that it could eventually lead to “tens of thousands” of new television transmitters.
Weiss was unsure how the commission would balance the two interests.
“The broadcasters certainly are providing a service to some proportion of the population, as evidenced in the Wilmington case,” he said. “The other interests claim that they want to eliminate broadcasting from that spectrum within five years and that they want to increase their power from 40 mW to 10 W. They say they will keep coming back to the commission until they achieve this.”
Weiss cited the success of a five-year DTS test in Pennsylvania that allowed WPSU-TV, a PBS member station owned and operated by The Pennsylvania State University, and licensed to operate in Clearfield, Pa., to reach an underserved area.
“The station was blocked from providing service to its full market,” Weiss said. “Now that’s possible.”
James E. O’Neal has more than 50 years of experience in the broadcast arena, serving for nearly 37 years as a television broadcast engineer and, following his retirement from that field in 2005, moving into journalism as technology editor for TV Technology for almost the next decade. He continues to provide content for this publication, as well as sister publication Radio World, and others. He authored the chapter on HF shortwave radio for the 11th Edition of the NAB Engineering Handbook, and serves as editor-in-chief of the IEEE’s Broadcast Technology publication, and as associate editor of the SMPTE Motion Imaging Journal. He is a SMPTE Life Fellow, and a Life Member of the IEEE and the SBE.
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