By Lisa Johnston
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The Federal Communications Commission (FCC) might be able within a few weeks to approve the compromise proposal by iBiquity and National Public Radio (NPR) to boost the digital output of digital FM stations, said NPR Labs executive director Mike Starling.
If the FCC delegates the issue to its media bureau, as seems likely, the issue won't have to wait for a vote by the FCC commissioners, he said.
Once the bureau acts, a small minority of FM stations currently broadcasting a digital signal could use their existing gear to boost digital power output, but many others — probably a majority of digital FM stations — would need to purchase a new transmitter, he said.
The compromise will extend the range of digital FM stereo signals and improve in-building penetration of digital stereo signals while minimizing a digital broadcast's potential interference with the analog signals of first-adjacent stations, or those operating on an adjacent frequency in “nearby but somewhat-distant markets,” Starling said. The compromise includes technical criteria, based on NPR field research, to manage a station's power increases to limit such interference.
The compromise, Starling added, “will preserve and energize the momentum behind the digital radio transition by helping ensure American listeners have the same ubiquitous relationship with digital service as they have with analog service.”
Under the compromise, all FM stations will be allowed to boost the output of their digital broadcasts from the currently allowed 1 percent of authorized analog-signal output to 4 percent, or a gain of 6dB. These stations could supplement gaps in digital coverage with low-power digital boosters, which are small enough to be installed on rooftops like small cellular base stations. Such boosters could be available in one to two years following development work by iBiquity and NPR, Starling said.
About two-thirds of FM stations could boost output beyond 4 percent of authorized analog output by implementing a so-called “asymmetrical power increase,” an approach that iBiquity and NPR have pledged to co-develop. These stations could increase the power of one of their two redundant digital signals to 4 percent of analog output and the other digital signal by anywhere up to 10 percent of analog output, Starling explained. Each of these redundant signals operates on a different sideband frequency. An asymmetrical 10 percent increase would improve digital reception but not as much as a 10 percent increase on both sidebands, which was originally sought by iBiquity.
If stations boost both of their digital sideband signals to 4 percent of analog strength, the range of their digital stereo signal would exceed the range of their analog FM stereo signal, Starling said. Currently, at an output level of 1 percent of analog output, digital FM stereo signals cover only about 89 percent of the footprint of an analog FM stereo signal, Starling said.
With a 4 percent boost to both digital signals, he noted, digital stereo range would be comparable to the range of “a good listenable mono analog signal,” whose range exceeds the range of an analog stereo signal, Starling added.
Also under the compromise, a small minority of FM stations would be able to boost their digital signal on both sidebands to 10 percent of currently authorized analog-signal output because, based on distance and terrain, they wouldn't interfere with the analog signals of first-adjacent-channel stations, Starling said.
NPR's estimates of current digital stereo range are more conservative than those previously mentioned by digital-radio developer iBiquity to TWICE.
In a past filing with the FCC, iBiquity said automakers and receiver manufacturers have expressed “concern about digital coverage and consumer reactions to products that may not have the same coverage as analog radio receivers.” These issues “continue to provide impediments to the successful rollout of HD Radio broadcasting.”
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