A recent proposal to increase the power levels at which HD Radio stations operate – to improve coverage and, perhaps more importantly, building penetration so that people can receive digital channels inside buildings – has been the subject of a cautionary study released by National Public Radio. That study was summarized in a story in the NPR magazine Current (an executive summary can be found here, and the entire 280 page study is here). The study agrees that an increase in power suggested by the recent proposal would increase HD Radio coverage and significantly increase building penetration, but it would do so only at the cost of causing interference to existing analog stations – in some cases significant interference. Such interference would be especially troublesome in receivers in cars, where radio broadcasters have long concentrated some of their most important programming to capture people in the place where competing entertainment options are most limited.
The NPR study does suggest that there could be ways to limit the interference using directional antennas or lessening power but using digital boosters that could be tuned slightly off-center on their frequencies to protect adjacent channel stations. HD radio operates on the sides of a station’s analog channel (thus its original name – "IBOC" for In-Band On-Channel), thus potentially causing interference to adjacent channel stations. By suppressing the signal on the side of the signal nearest to the adjacent channel station and sending the digital bits out of the other side of the channel, some of this interference could be minimized. Yet systems capable of such protections have not yet been fully developed.