An HD Radio in-band/on-channel (IBOC) hybrid radio signal includes both analog and digital signal components (see figure). One important aspect of the signal design is how well the digital portion of the signal replicates the coverage of the legacy, analog portion. Two proposed FCC rule changes, requested by the NAB and others, which promise to optimize digital coverage without creating objectionable interference are now being considered. Comments and replies on these proposals were due to the FCC by September 21, 2023 with replies October 6, 2023.

Image of PILOT radio test bed FM spectrum with three transmitters in operations.

Three FM-band IBOC signals spaced 400 kHz apart, where the center signal at 98.1 MHz is second-adjacent to each of the other two signals. Each signal has an analog component and two digital sidebands, an upper and a lower. (Image of PILOT radio test bed FM spectrum with three transmitters in operations.)

The FM digital power NPRM, MB docket no. 22-405, proposes changes to the digital audio broadcasting (DAB) technical rules that would allow more stations to increase their digital power over the current blanket authorization value of -14 dBc (decibels below the unmodulated FM carrier, a measure of digital signal power), by relaxing the formula used to assess the potential for objectionable interference to first-adjacent channel neighbors. The new formula, proposed jointly in an October 2022 petition to the FCC by NAB and HD Radio developer Xperi Corporation, is based upon years of experience obtained in operating FM-band HD Radio signals, and was widely supported by broadcasters and others when published by the FCC for comments in late 2022.

There is ample evidence to show that increased digital signal power can increase coverage. Xperi (then iBiquity Digital Corporation) conducted field tests back in 2007 to illustrate how increasing digital power from -20 dBc (then the blanket authorized digital signal power) by 10 dB to -10 dBc (the maximum digital signal power currently allowed) would improve a station’s coverage area. The two maps below show the coverage for digital signal power of -20 dBc (left map) and -10 dBc (right map). Note in particular the test routes with arrows pointing to them, showing the improvement in coverage (portions of the route now received in digital due to the higher power level).

Digital coverage map for WJRZ-FM showing coverage with digital sideband power of -20 dBc (left) and -10 dBc (right). Arrows point to areas of interest where coverage was improved in the -10 dBc case. (Source: iBiquity Digital Corporation)

More recently, NAB and its partners conducted a field test in 2021 to demonstrate that the new formula would not result in objectionable interference to first-adjacent channel stations. The field test project and results obtained are highlighted in a technical paper submitted at the 2023 NAB Broadcast Engineering and Information Technology Conference (BEITC), “High Power Digital FM Field Test Project,” authored by David Layer, NAB; Alan Jurison, iHeartMedia; Russ Mundschenk, Xperi; Steve Shultis, NYPR and E. Glynn Walden, Audacy. This paper, as well as 34 others from the 2023 conference, are available to purchase here.

The field test described in this paper involved recording audio from an analog FM signal in the presence of first-adjacent channel IBOC signals, first at -14 dBc and then at -10 dBc, using four different receivers and two different test stations. These audio recordings were subjectively evaluated by two different listener groups and the results, shown in the table, showed that the audio quality of the analog signal was essentially the same whether the neighboring IBOC station operated at -14 dBc or -10 dBc.

Summary of subjective evaluation results obtained when listening to audio recordings obtained in the field of an analog FM signal with either a -10 dBc or a -14 dBc IBOC 1st-adjacent channel interferer.

The second change to the rules proposed in the FM Digital Power NPRM would allow FM broadcasters to transmit the digital portion of the signal asymmetrically, with different power levels for the lower and upper digital sidebands. This rule change was proposed jointly by NAB, Xperi and National Public Radio in December 2019, and would allow a digital FM station to protect, for example, an analog FM station on a lower first adjacent channel, while enabling an increase in digital power on the upper sideband where there is no adjacent analog FM station or a more distant adjacent station.

Currently, broadcasters are allowed to operate asymmetrically but to do so, an experimental authorization request must be filed with the FCC and is subject to periodic renewal. As with the power increase proposed in 2022, the asymmetric sideband proposal has been widely supported by broadcasters and NAB is hopeful that both changes will be adopted following the comment and reply comment deadlines.

Broadcasters or any interested party can easily file comments or replies using the FCC’s online Electronic Comment Filing System (ECFS) here, specifying proceeding 22-405. Again, comments and replies were due to the FCC by September 21, 2023 with replies due October 6, 2023.