NAB and Xperi question FCC cap on FM digital power
A February 2025 FCC grant letter says the total digital sideband power of an FM station using extended HD Radio modes may never exceed 1.75 kW, or –10 dBc. NAB and Xperi say this departs from how the commission handled such grants before, and they warn it could shrink digital coverage.
The Boise letter
The letter went to KBSU(FM) in Boise, Idaho, which had asked for increased digital power. NAB and Xperi used it as their example when they met the FCC Media Bureau on Oct. 29.
For the extended hybrid modes, the commission wrote that the licensee "must adjust the total digital sideband ERP in accordance with our rules." It added that the total "may never exceed 1.75 kW (–10 dBc)."
The two groups say the FCC followed NRSC guidelines for more than a decade instead. Under Table 4-4 of the NRSC standard, a station authorized at –10 dBc that ran the MP3 mode could use a total of –9.2 dBc, a slightly higher level. In their account, that margin existed so the added subcarriers would not push down the power of the primary ones. They describe the 2025 wording as an apparent change.
What –10 dBc and the service modes mean
Xperi is the parent company of HD Radio, the authorized U.S. hybrid digital radio standard. A station sending its digital sidebands at –10 dBc runs them 10 decibels below the analog carrier's effective radiated power, which is 10%. At launch the digital limit was –20 dBc, or 1% of analog power.
Most U.S. stations use the standard MP1 mode, with 96 kbps across 10 frequency partitions per sideband. Extended modes such as MP3, at 120 kbps, and MP11 place extra digital subcarriers closer to the host analog carrier. Those modes are where the disagreement sits.
Coverage and interference arguments
The joint presentation warned that holding total digital power fixed while adding subcarriers forces each subcarrier to carry less power. The parties said that could mean reduced digital coverage and more frequent blending from digital to analog in fringe areas.
They also argued that the higher totals in the NRSC table cause no harmful interference to first-adjacent analog stations. The extended subcarriers sit inward, toward the station's own analog signal. That puts them farther from adjacent channels, and the parties believe receiver IF filters attenuate them.
The question is already before the commission in another form. In 2024, NAB and Xperi petitioned it to update the rules to reflect this system design and to match the grant letters issued before 2025.
The meeting and the aviation track
Xperi's Jeff Detweiler and Paul Peyla, Ashruf El-Dinary for the National Radio Systems Committee, and NAB's David Layer and Larry Walke met Media Bureau officials. They asked whether FM hybrid digital broadcasts are truly capped at a total of –10 dBc. A Nov. 2 ex parte filing describes the meeting.
Layer also updated the bureau on talks between commercial broadcasters and aviation stakeholders studying possible HD Radio effects on the aeronautical radio navigation spectrum. The Air Line Pilots Association International had earlier raised concerns about interference with pilot navigation systems from 108.0 to 117.975 MHz. As a result, stations at 107 MHz and above need an experimental authorization if their IBOC signals use asymmetric power levels. NAB says aviation representatives have completed compatibility testing and plan to meet NAB and Xperi within the coming month to review the results.
Our view
The open point is whether the KBSU wording is a new FCC position or the language of one letter. A Media Bureau answer to the Nov. 2 filing would settle that, and so would action on the 2024 petition. Until then, stations that run extended modes cannot tell which reading applies to them.
Source: Radio World