Warp Factor 6, Mr. Scott... (The Ongoing IBOC Power Increase Debate)

October 21, 2009

With its Oct. 7 ex-parte presentation to the FCC on docket 99-325, NPR put to rest any doubts that it would oppose a 6dB interim increase in FM IBOC digital carrier power, based on its most recent study of digital-to-analog interference and input from member stations. Those attuned to the FCC Media Bureau's current vibe suggest that with NPR, Ibiquity, and the Joint Parties now in seeming harmony, the Commission will likely affirm the recommendation and HD hybrid signals with digital signals at 14dB below carrier will soon become status quo.

While the pending throttle-up isn't exactly a surprise, its reality will pose an interesting test to the financial commitment of those 20 percent or so of licensed FMs already having made the leap to IBOC. To wit, how many existing installations will have sufficient headroom to implement the increase without significant retooling of transmission hardware?

Some perspectives from transmitter manufacturers on the practical implications of a digital increase were discussed at the 2009 NAB Radio Show and summarized in a recent edition of NAB Tech Check. And what they had to say reveals that for a number of facilities, achieving that 6dB boost is going to involve a lot more than holding down the digital raise button.

Harris' Geoff Mendenhall pointed out that for those using low-level combining (i.e., common amplification of analog and digital carriers) higher digital carrier levels mean that the intermediate and final amplifier stages must be operated in a more linear (and inherently less efficient) fashion in order to meet the emission mask. Simply put, the higher the ratio of digital-to-analog power, the more a given transmitter will have to be de-rated. So as the digital side gets boosted, the transmitter's analog top-end output will suffer. While some installations may have enough reserve capacity to allow existing hardware to make 6dB more digital signal, amplifiers will have to be readjusted to operate in a more linear part of their curve, sucking up remaining analog headroom. Some simply won't make it with their existing transmitters, and plant cooling capacity will also have to be bumped up along with electrical power mains service, and back-up generating plant output.

Another interesting twist was added by Nautel's Gary Liebisch, who noted that transmitters operating in the FM reserved band below 92MHz tend to fall further behind in efficiency as linearity is increased than do those operating at higher frequencies. In other words, depending on what channel you happen to be on...your mileage may vary.

Those using high level combining schemes will face similar issues, as reject loads, digital transmitters, etc., will all have to be upsized.

Then there's also the matter of addressing those special situations where first adjacent channel stations may be short spaced, and even a 6dB increase in digital carriers may degrade analog coverage within the adjacent station's protected contour. NPR's presentation to the FCC calls for an enhanced remediation process where such circumstances occur, but there's no clear indicator of what such a provision might look like.

Interestingly, NPR has temporarily taken its Web-based IBOC digital carrier-to-analog interference calculator down to bring it into harmony with the new compromise interim level, but has left up a set of audio samples that demonstrate the analog impairment that can result from a -10dBc, or even a -20dbc digital carrier on an adjacent channel.

One thing's for certain: Approval of a 6dB HD digital boost will mean lots of extra hours, as managers and owners task their engineers with delivering a "wee bit" more digital drive without melting down already overheated corporate budgets. For many, actually making it happen is likely to be an agonizing process.