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Thirsty for power

ATSC calls for increased digital signal strength 4/22/2001 08:00:00 PM Eastern

After a year of meetings, an Advanced Television Systems Committee RF task force has decided that the "main limiting factor" to reception is adequate field strength at a given location, not the modulation standard.

In plainer language, that means a station's digital signal must be strong enough to be received, whether that signal is being transmitted indoors, to a cell phone or via pager or wristwatch.

That may not sound like rocket science, but significantly, the task force report stressed, for the typical home viewer to receive adequate indoor reception with 8-VSB, TV stations must boost the power of single transmitters or build on channel-repeaters to get a signal to consumers in hard-to-reach fringes of their coverage areas.

In its final report, entitled "Performance Assessment of the ATSC Transmission System, Equipment and Future Directions," the task force last week also concluded that newer generation 8-VSB receivers appear to meet the goals of outdoor reception by fixed receivers, in accordance with FCC DTV planning factors.

This data on third-generation 8-VSB receiver chips echoes a separate report issued last week by the FCC's Office of Engineering & Technology. (BC, April 16, page 12).

The need for stronger signals could raise issues about whether a station's FCC-allotted power level is adequate to serve its market, but Mark S. Richer, ATSC executive director, disagrees.

"The point is not whether the FCC has allocated enough power in a given market for a given station, but whether you are getting that signal strength into various areas from a single point of transmission," he explains. "For example, if you are operating in an urban environment and the signal is being shielded, you may need an on-channel repeater to boost the signal.

"The question is whether you can build a transmission facility to deal with that," says Richer. "A lot of the problems with reception of DTV have been blamed on the modulation standard, but it really has nothing to do with that. It has to do with signal strength. That's also true for any kind of pedestrian or mobile application.

To solve this, the ATSC is suggesting that stations may have to create a cellular-style network of numerous transmitters, an expensive solution.

"Broadcasters are going to have to look at the cost of the build-out to do that," Richer warns.

For conventional TV reception with rabbit ear and other indoor antennas, a handful of repeaters in a station's market would be sufficient.

Skeptics of the report point not only to the cost but to the availability of multiple frequencies that an on-channel system using 8-VSB would require. Bon Abercrombie, senior executive vice president of planning and development for Pappas Telecasting (and a proponent of COFDM modulation), says that, to his knowledge, you can not transmit on a single frequency with on-channel repeaters.

"I've not had a chance to read the entire report, but, from what I'm hearing, it says what we've known for months that 8-VSB only works as a fixed service with an outdoor antenna. That's the bottom line. We still think that COFDM is by far the better choice."

—Bill McConnell contributed to this story.

 

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