The Federal Communications Commission this month will vote on a change to streamline the process for allowing FM digital transmissions at different power levels on upper and lower digital sidebands, known as asymmetric sideband operation.
The change, if adopted, is intended to help stations improve digital coverage while minimizing the risk of harmful interference to other stations. The commission will exclude stations on Channel 300, otherwise known as 107.9 MHz, the top end of the FM band.
Meanwhile the commission has chosen to defer action on a separate but related proposal to allow digital FM stations to use the highest allowable power levels.
In an FCC Fact Sheet announcing the planned vote at the September meeting, the commission says the order will authorize FM stations broadcasting digitally to adopt asymmetric sideband operation without the need for experimental authorization.
The HD Radio FM digital radio system inserts redundant digital sidebands above and below a station’s existing analog signal, according to the FCC.
Digital sidebands typically operate using the same power levels. With asymmetric sidebands, an FM station uses different power levels on each sideband. The technique provides the station with additional flexibility to protect adjacent-channel stations from interference and to maximize digital signal coverage.
“Overall this will streamline the notification process for digital FM operations,” the FCC said in the release.
The order would adopt a table that FM broadcasters can use to determine the maximum allowable power of each digital sideband. It would also clarify that certain notifications pertaining to FM digital operations are to be made using existing Form 2100, Schedule 335-FM, including: the initiation of hybrid digital operation; the initiation or discontinuance of asymmetric sideband operation at any power level; and certain digital power increases and decreases.
The commission said it is deferring action on a proposal to change the methodology for determining a digital FM station’s allowable power, which would allow more stations to increase digital power levels. That proposal appeared to stall after several aviation groups expressed concern about the potential for interference on navigation systems.
The Aerospace Industries Association (AIA) and the Air Line Pilots Association International (ALPA) raised the possibility of interference between HD Radio stations at 107.9 MHz and the adjacent Aeronautical Radio navigation Service (ARNS) band operating from 108.0–117.975 MHz.
Radio World reported at the time that Xperi, the developer of HD Radio, and the National Association of Broadcasters had engaged in “constructive dialogue” with aviation and aerospace stakeholders over technical matters.
In the draft of the pending report and order on asymmetric sidebands, the FCC wrote, “The commenters state that preliminary industry studies suggest that further testing is needed; to that end, aviation industry commenters are working with NAB and Xperi, but state that comprehensive testing will take additional time.”
The commission’s monthly meeting is scheduled for Sept. 26.
