There has been an email conversation within the public radio community about whether or not to increase the power of the digital ("HD Radio") service of those radio stations which provide both analog and digital services. Unlike digital television, which completely replaces analog television next February and for which digital operates on a completely different channel, digital radio operates on the same channel, magically intertwining with the analog signal. It's not likely that analog radio will go away any time soon, or perhaps ever.
So the fact that these two technologies have to co-exist has led those who set conditions for such things to provide for a very low power level for the digital component. But that has caused HD Radio coverage to be, in many cases, less than for the analog part of the signal. In public radio, some stations, such as the ones I managed out in the Northwest, take advantage of nearby mountaintops for transmitter sites, but that often means they're not as closely located to the metropolitan population center as is desirable in a low-power digital situation.
The FCC has been looking into this and is considering permitting stations to increase the digital power level. NPR has conducted tests on this and it has revealed some concerns. The Executive Director of NPR Labs, Mike Starling, takes over from here in a note he posted yesterday to the public radio system:
Regarding the recent messages about HD Radio coverage and the proposed power increases, our studies show we need to balance the need for digital coverage improvement against potential interference to your analog signals.
While improving HD Radio indoor coverage is essential, NPR supports a managed power increase, on a voluntary basis, where needed. We do not support iBiquity’s unconditional, proposed 10 dB power increase at the minority of stations where our studies predict interference can occur to more than 20% of the listeners INSIDE of the protected 60 dBu contour of neighboring first-adjacent stations.
Please join us next week for an interconnect to go over this issue and why we feel so strongly that a managed increase is necessary. We’ll be providing audio samples of the interference we found in our study -- so you can actually hear the interference effects for yourself.
[Here, I take out the interconnect info, but if you're in public radio, please check for the details with your A-Rep and join the discussion. --Dennis]
The FCC has put both the NPR Labs DRCIA report and the “Joint Parties” filings out for public comment, with a comment deadline of November 28 (yes, the day after Thanksgiving). We are eager to hear from you so that your views will inform our comments in this proceeding - and to hopefully achieve a common public radio position on this filing.
- DRCIA Report - Executive Summary http://www.nprlabs.org/publications/reports/200807151044-DRCIAFullReport-ExecSummary.pdf
- HD Interference Maps showing expected FM interference coverage for 49 stations: http://www.nprlabs.org/research/drcia_maps_index.php
While adjacent channel IBOC interference does not obliterate the signal or fundamentally alter basic intelligibility, it increases the noise level of reception, which is most noticeable on voice programs such as news and public affairs. In our report, a consumer listener panel determined our protection criteria for the FM signal at not less than “good” audio quality. While it is still a listenable signal for committed listeners, it is audibly noisier than interference at the present 1% IBOC power. And outside your protected coverage area (which the FCC has recently reminded us they do not protect against such effects) it would be worse. We are especially concerned in situations where adjacent channel spacing is insufficient in our view to support a full 10dB power increase among spectrum neighbors (which could include your station!).
Even in close-spaced instances, we believe alternative solutions such as unequal sideband power, directional IBOC antennas, and single frequency network boosters can provide the needed improvement in indoor signals without generating the interference levels that listeners judge as degraded signals. To our favor, iBiquity announced at the September NAB Radio conference that they will fully support the development and deployment of these recommended alternative strategies.
Just as our DTV colleagues faced significant challenges a few years back with the need for increased power, we hope this chapter in HD’s rollout will be chronicled under “early lessons learned” about the robustness and indoor penetration limits of digital signals.
Hopefully, this too will turn out to have been an important, but ultimately successful turn of events to optimize the digital deployment for the future. We look forward to hearing from you during the interconnect.
Some very interesting trade-off decisions to be made. Update 14 November 2008:
Recommend you also read the comment from PocketRadio below, from which this link to Cost considerations for increasing HD Radio another 10 dB comes. --Dennis
Some very interesting trade-off decisions to be made.
Update 14 November 2008: