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FM Stations Seek HD Radio Power Boost

FM stations broadcasting a digital HD Radio signal should be allowed to boost digital-signal output to extend range and improve in-building penetration, major broadcast groups and transmission-equipment makers told the Federal Communications Commission (FCC).

At current power levels, the group said in an FCC petition, “a significant number of potential listeners are unable to benefit from digital broadcasts.”

HD Radio developer iBiquity Digital clarified the petitioners’ assessment by pointing out that digital FM stereo coverage already exceeds analog FM stereo coverage by about 10 percent. The digital FM stereo footprint, however, reaches only about 80 percent of the footprint of an FM station’s monaural analog signal.

With the power boost, an iBiquity executive told TWICE, “Digital FM stereo coverage will equal monaural analog FM coverage. We’ll deliver digital stereo farther than analog stereo.” In general, he added, the 10dB maximum boost proposed by the group would boost a digital signal’s range by about 10 miles.

Indoor digital coverage will also get a boost. In their filing, the petitioners said their tests showed that the requested power increase “resulted in digital-signal building penetration equal to or exceeding analog performance” and that as a result, the power boost would deliver “digital indoor reception with better quality than analog.”

As a result, a consumer who now gets noisy analog mono signal indoors will more likely get a digital stereo signal indoors if a station is allowed to boost output, iBiquity added. Wood buildings reduce signal strength by about 7dB, the company noted, and brick cuts strength by about 10dB. If a radio that got analog stereo reception outside a building went indoors and suffered a 10dB signal drop, “I probably lost analog stereo inside, and I might lose the digital signal altogether,” the iBiquity executive explained. “Now, I’ll probably keep the digital signal inside.”

It will be about a year before the FCC votes the petition up or down, although it could take longer, iBiquity said. The FCC’s next step is to authorize additional field testing to supplement field tests conducted by the major broadcast groups and equipment makers. After the next round of tests is completed, the FCC will collect comments and then vote to change its technical rules.

The FCC currently limits the strength of FM stations’ digital broadcasts to 1 percent of their authorized analog-signal strength, but the group petitioned the FCC to power up digital FM signals to 10 percent of analog signal strength. Based on field tests, the group concluded, the boost would enable “FM stations to improve their digital performance with minimal risk of harmful interference to existing analog service.”

The 1 percent cap, the petition said, was “intentionally set at a very conservative level to ensure that the introduction of digital audio broadcasting would not adversely impact analog compatibility. The relatively limited on-air testing of the IBOC [in-band on-channel] system that had been conducted at that time justified such a hyper-conservative approach.” Since then, the petition continued, experience has shown that the low digital FM power level “precludes many of these stations from being able to transmit a digital signal that fully replicates their analog service areas. The lower digital power level also limits the ability of digital FM signals to penetrate buildings and other structures, thereby impeding indoor reception. Consequently, a significant number of potential listeners are unable to benefit from digital broadcasts.”

With the filing, “broadcasters and equipment makers clearly say it’s time to increase the power devoted to FM HD Radio broadcasts,” HD Radio developer iBiquity Digital said. “The initial HD Radio rollout was structured to ensure minimal interference of analog signals while also offering a strong digital service. That first phase has gone smoothly, with minimal interruption of traditional radio broadcasts.” The test program, iBiquity added, determined that the proposed increase in digital FM output would significantly improve digital FM performance with virtually no risk of adversely affecting analog compatibility.”

The tests found little impact on the host station’s analog signal or the analog signals of FM stations operating in nearby markets in adjacent frequencies [first- and second-adjacent stations], iBiquity said.

The petitioners consist of 18 broadcast groups operating 1.212 commercial and noncommercial FM stations, representing 13 percent of all FM stations nationwide. The equipment makers are the four largest makers of radio transmission equipment, the petition said.

More than 1,700 AM and FM HD Radio stations are on the air.