Media Bureau seeks comment on asymmetric sideband operation that would permit HD operation with different powers on different carriers
As the HD Radio slogan says (see illustration to the left), it's time to upgrade . . . again. In the latest effort to get HD radio’s actual performance to come close to its original promise, some of the system’s cheerleaders have advanced a new approach: asymmetric sideband operation for FM stations. And less than a month after the idea was pitched to the Media Bureau in an ex parte meeting, the Bureau has invited comments on a couple of reports that were left behind after the meeting. While it’s still a bit early to say, there’s reason to believe that this could be a happening thing before too long.
First, some technical background on HD radio. FM radio stations that transmit hybrid analog and digital signals do so by placing digital carriers on the first-adjacent frequency on each side of their licensed analog channel. For example, a station whose analog signal is on 97.1 MHz and places digital carriers on 96.9 and 97.3 MHz. The existing rules require both digital carriers to be at the same power level (-14 dB with respect to the analog carrier).
The twist in the asymmetric sideband proposal: a different power level would be permitted on each digital channel.
Digital signal levels are far below analog levels. That’s partly because digital signals need less power generally. But more importantly, it’s because the digital carrier signals on the transmitting station’s adjacent channels pose an interference threat to other stations. Stations operating under the FCC’s original digital rules (with power levels limited to -20 dBc) found their coverage to be inadequate. The FCC responded by allowing increased digital power to – 14 dBc for most stations and – 10 dBc for stations that can show no interference. But some stations with “super-powered” status or that are short-spaced to adjacent-channel stations have not been able to take advantage of maximum digital power levels.
Recent experimentation conducted on a few FM stations under special temporary authority indicates that digital reception improves with a power increase even if the increase is on only one of the two digital frequencies. Thus, in our 97.1 MHz example, it might be OK to increase digital power on 96.9 MHz, even though an increase on 97.3 MHz would unduly interfere with another station. Of course, the results may not be optimal. As one of the reports concluded, “broadcasters are best off maintaining symmetrical sideband levels” – but “coverage improvements are possible with an increase of only one sideband.”
After chatting with iBiquity reps and some of their supporters and looking at test results, the FCC has decided to invite comments on regularly licensing asymmetrical digital operation for those who want it. Comments will be due 21 days after the Public Notice is published in the Federal Register; reply comments will be due 14 days later. (Check back here for updates on those deadlines.) Historically, the Commission has tended to be very accommodating when it comes to HD radio, so anyone interested in HD radio – whether as proponents or nay-sayers – should pay attention here, as this looks like it may move quickly.