Engineers Set Out to Test Distributed Transmission System Limitations

Distributed Transmission System technology has become highly politicized since the CTIA and CEA suggested that it could be used to reclaim some of the spectrum used in the TV band, industry executives and engineers said. The technology, also known as a single frequency network, lets stations use multiple synchronized transmitters to supplement the one on their main tower. It was approved by the FCC in the lead-up to the analog cutoff. Broadcast executives have panned the technology and the criticism is growing. Now a group of engineers is setting out to prove that the DTS won’t work. The CTIA-CEA proposal “did not fully understand the application, and that is what has gotten some of the broadcasters’ backs raised over this whole thing,” said Jay Adrick, vice president of broadcast technology for Harris, which sells some equipment for DTS.

The problems with a multiple transmitter architecture for ATSC DTV broadcasts can be traced to choices made when the original ATSC DTV standard was adopted, said Oded Bendov, a broadcast engineer who recently laid out his concerns in a white paper (http://xrl.us/bgxwnv). Because the ATSC standard can’t handle multipath signals as well as competing overseas DTV standards can, the addition of multiple transmitters can overwhelm a receiver, he said. “That is an inherent problem of the ATSC standard,” Bendov said. “It was not designed with multipath in mind. The rest of the world designed a standard primarily with multipath in mind.”

In COFDM systems such as FLO and DVB, multipath signals reaching the receiver can be combined to produce a stronger signal, said Mark Aitken, director of advanced technology for Sinclair Broadcast. But with ATSC, the receiver picks one signal to focus on and ignores the rest.

Bendov and others will try to prove the system’s technical merits through a simulator, he said. “There’s no independent rigorous testing of any distributed transmission system from the time it was first proposed” in the public record, Bendov said. Though some companies have tried it, none have adopted it fully, he said. “If it worked, other people would implement it. I'm not saying there won’t be others who try it. But I would stake my reputation that our testing will show that DTS is not a replacement technology for broadcasting and it will not rescue mobile DTV.” After initial tests, the simulator will be opened to the FCC and others interested, Bendov said.

DTS may be useful in two cases, Bendov said. It can be useful in providing service to a geographically remote region within a station’s licensed area, such as a community separated from the main transmitter by a mountain range, he said. The other use is in improving coverage in some places at the expense of creating interference in others, he said. “DTS or SFN based on the ATSC standard is simply not a viable technology except for those two cases,” he said.

Not all engineers are as skeptical about the technology, though many worry about its cost. Some vendors have been pushing DTS for mobile DTV applications, and others say they recognize the benefits it has in certain situations. “I think DTS is a great idea, but we think it’s a limited application and it’s not a broad brush type of service,” said Don Everist of Cohen, Dippell and Everist environmental consulting.

ATSC has real shortcomings for DTS, and a cookie-cutter approach to implementation won’t work. But it can be successful if properly engineered, said Merrill Weiss, a broadcast consultant who works on DTS issues. “You have to start with the way receivers work, but in the end, all of this comes back to what do you do with transmitter placement and antenna patterns,” he said. “There is a whole range of issues that have to be thought through carefully to design a single frequency network using ATSC, which admittedly isn’t the best choice for SFNs, but it’s what we've got."

Using DTS for mobile could work technically, but it may be prohibitively expensive, said Brett Jenkins, Ion Media vice president of technology. “If for some reason we really needed DTS solutions” to make the mobile DTV “business work, I would feel pretty confident we could find the right technical solutions to make it happen,” he said. “But I don’t see it as a requirement out of the gate."

The political attention to the technology hasn’t affected broadcaster’s interest in it, Adrick said. “There’s no decrease in interest and there’s no increase in interest,” he said. “The full power broadcasters view this as something that they will probably end up doing in many cases in the future once mobile becomes a major portion of their service."

Still, broadcasters haven’t been rushing to build the systems. Only 18 stations have filed DTS applications or other paperwork with the FCC since it adopted DTS rules, commission data show. And those applications seem stalled, Everist said. “The commission has got 10 or 20 applications for this and they're not moving,” he said. “We have five applications on file an we'd like to know where they are.”