Squeezing Blood From the FM-HD Stone

It has long been understood that HD Radio signals do not play well with others. The digital sidebands of an HD transmission have the potential to cause interference to nearby radio stations. The problem is most notable on AM, but there’ve been issues with FM-HD as well.
Initially, digital FM sidebands were broadcast at 1/100th the power of their analog “host” signal. The weakness of the digital signal caused all kinds of reception difficulties. After years of wrangling, the FCC approved a ten-fold increase in digital sideband power in an attempt to make the signals more robust.
However, this “fix” cannot be applied uniformly to all HD broadcasters. Those in spectrally-congested areas of the country risk causing increased interference to their neighbors on the dial, and many stations simply don’t have the overhead to feed the necessary power into their transmission systems.
Thus, HD Radio’s proponents have been experimenting with more nuanced ways to improve reception of an FM-HD signal.
The first involves raising the HD broadcast power of a station asymmetrically: boosting the power on only one of a station’s two digital sidebands. The idea is that, in areas where adjacent-channel interference may be a problem, a station could raise the power on the sideband furthest away from its closest neighbor on the dial.
NPR Labs, on contract to transmitter manufacturer Nautel, released a report in June that suggests asymmetrical digital power increases can improve the reception of FM-HD signals, and that such tweaking causes “little or no measurable impact on the performance of analog test receivers.”
But the sting is in the report’s final paragraph: “In sum, broadcasters are
best off maintaining symmetrical sideband levels, but coverage improvements are possible with an increase of only one sideband.” In the end, stations that elect to increase their FM-HD power will pay for it either way. An asymmetrical power increase may be a neighborly gesture in congested broadcast markets, but it’s no magic bullet for improving digital FM reception.
The other option for beefing up an FM-HD signal involves the deployment of Single Frequency Networks (SFN). This is a fancy name for a digital booster station. HD proprietor iBiquity, with funding from the NAB and assistance from broadcast-investor Greater Media, set up two SFNs in Baltimore and Boston, each located about 20 miles away from their parent radio stations.
Last month, iBiquity reported that SFNs could “provide broadcasters with the ability to selectively extend digital coverage…without compromising the existing HD Radio digital service area.” However, it also conceded that SFNs had the potential to degrade the reception of a station’s analog signal, especially as one got within a mile of the SFN transmitting site.
The deployment of digital boosters raises some uncomfortable questions. Are FM radio stations willing to sacrifice any portion of their analog coverage area to beef up reception of their HD signal? More importantly, are they willing to build new broadcast infrastructure to reach the pittance of listeners who actually have HD receivers?
On balance, the potential “improvement” to FM-HD through either of these techniques is marginal. This is precisely why broadcasters who have adopted the technology now use questionable analog means to try and recoup their investment.