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For AM IBOC, The ‘Real Test’ Comes Now

“The real tests come now.”

“The real tests come now.”

That’s how one engineering group head characterized what will happen now that digital AMs are allowed to leave their digital transmitters on at night.

The FCC rules authorizing IBOC become effective Sept. 14. The rules give certainty to many aspects of the technology; and for AM owners — many of whom hope the promise of better audio quality gives them parity with their FM counterparts — it’s been a long wait.

“As more people turn it on at night, it will be interesting to see who gets interference, who’s wiped out and who’s not,” said the group engineering executive, asking not to be identified.

The National Radio Systems Committee reviewed test results conducted by Ibiquity Digital several years ago to predict possible AM nighttime interference, but its leadership said at the time there was no practical, consistent way to measure the effect of IBOC operation on skywave reception because propagation can change each night due to varying atmospheric conditions.

Engineers said then that the only real way to test AM nighttime IBOC was to see what happens when several stations in a market light up.

No ‘mad dash’

Now that presumably will happen. But how quickly?

A big question is the pace of the rollout. Several radio group engineers contacted for this article don’t expect a mad dash for AMs to go on at night. They believe large-market, big-wattage clear-channel stations that have already made their nighttime transmission equipment preparations will transmit digitally at night first.

One of those 50 kW high-powered stations is Buckley Broadcasting’s WOR(AM) in New York. Director of Engineering Tom Ray, a vocal IBOC advocate, said he’s ready to go digital at night.

“We have been ready for a long time. We have a bunch of listeners in HD. We can’t wait to have it on at night and light up our audience.”

Ray, who lives in the null of his station’s antenna pattern, plans to listen to his stations and others on the night of Sept. 14. “It will be interesting to drive around and see what happens.”

He’s curious about the HD reception in his area of other high-powered AM clear-channel stations like WLW in Cincinnati and WGN in Chicago.

Clear Channel Radio was still formulating its nighttime digital plan when contacted in August. Steve Davis, senior vice president for engineering and capital management, said, “Our plan will be to begin nighttime operation on all those AMs under our control which can do so within the prescribed mask in a manner reasonably conforming to our analog patterns. It will be an exciting time to truly evaluate this technology.”

Throughout the IBOC conversion process, he said, “We have been measuring our arrays, calculating pattern bandwidth and making other engineering assessments with respect to the feasibility of in-mask compliant nighttime operation on our AM stations, along with assessing the feasibility and practicality of daytime operation. For some AM stations this simply won’t be possible, due to physical factors such as the geometry of the array or tower heights.

“Likewise we have AM stations that we haven’t converted to IBOC, even during daytime hours, for the same types of reasons,” Davis continued.

“For other stations it will be possible, but only with extensive rework such as a new phasor, which may not happen for some time.”

Instances in which a station has a single, non-directional tower with a currently compliant daytime HD-R operation are poised to go nighttime digital, according to Davis.

Crawford Broadcasting Director of Engineering Cris Alexander, who like Tom Ray contributes to RW, agrees with Davis that stations that have disparate day/night antenna patterns with different power levels have some work to do before they can turn on their digital at night.

“A different nighttime pattern means a different set of parameters, so all the adjustments you did for daytime have to be redone for night,” he said.

Of its 15 AMs, Crawford has converted 12 to IBOC. Out of those, four are ready to go nighttime HD-R.


Cox Radio Orlando Director of Engineering Steve Fluker said, “Personally I don’t think the adoption of AM IBOC at night will create a flurry of station upgrades.

“I do think though that most of the stations already operating during the day will welcome the chance to leave it on 24 hours a day. I think we have such a division out there where stations are either totally committed and excited about AM HD and are already in the process of converting, or they aren’t behind it yet and are taking a wait and see approach before making the investment.”

Several engineers, including Fluker and Glen Clark of Glen Clark & Associates, an AM engineering consultancy, said many smaller AMs that are holding back on converting to IBOC at all, much less at night, are doing so due to cost reasons.

Owners should be taking better care of their directional arrays, said Clark. “Ibiquity has a graph that says ideally your VSWR should be less than what’s shown in this graph if you want to have good HD performance. Few stations today meet that spec.”

Clark has seen some stations that have narrowband antennas “clean those up,” but he believes when people realize the correlation between antenna bandwidth and service area, “that’s when serious money will be spent on broadbanding.”

Fluker agrees that, with AM IBOC antenna systems and especially directional arrays, “It’s not advisable to just install the digital exciter and turn it on. The antenna tuning system must be checked first and properly tuned for the digital signal. This goes for both ND and DA systems. Also tuning does not just mean adjusting the match to 50 ohms J0. There is a critical phase match between the antenna and transmitter that must be checked and corrected if necessary.”

An improper match, Fluker said, “can cause the digital sidebands to become less symmetrical across the carrier which will make them more noticeable on AM analog radios. This can be heard as a low hiss, buzz or sort of a ‘bacon frying’ sound. Properly tuned, these artifacts can virtually go away.”

This noise can also be aggravated on wide-band AM radios, he said, such as some that had been carried earlier, but are no longer, by Chrysler and Mercedes.

Opinions are all over the map when it comes to predicting interference on AMs at night to adjacent analog channels from stations broadcasting in IBOC. Several engineers said much of the interference will occur outside a station’s FCC-protected contour, and therefore, its listening area.

Ray said Internet discussions came alive after the IBOC rule effective date was published in the Federal Register, and many of those who object to AM IBOC at night do so because of the technology’s assumed diminishment on skywave listening.

“They’re talking about DX listening. I don’t serve the DXer. I serve the local community.”

Null areas tricky

Here’s the rub interference-wise. AM directional antennas present more of a challenge than do non-DAs, observers said. Most directional arrays are adjusted and tuned at the carrier frequency and as the bandwidth is increased, the characteristics of the antenna system can change.

“You may even see some slight changes in the null areas in the different areas of the bandwidth,” said Fluker.

“These changes with only the analog audio were not too objectionable, but you might hear the quality of the audio change as you drive through a deep null. Since the digital sidebands are even further removed, their characteristics in a null will behave differently from the main audio and therefore could create interference to their host channel in these nulls.”

Such host channel interference, said Fluker, could be more of a problem than interference to other adjacent stations due to skywave. After all, the HD signal still fits within the legal bandwidth of the AM station.

The engineer who wished to remain unnamed said if he were the owner of a small AM that already has marginal coverage, he’d be asking, “How far does the AM digital signal carry and what problems related to interference will crop up? How far do I get; is it usable or choppy?”

What happens if the FCC receives interference complaints? Could it change or rescind authority for some AMs to go IBOC at night?

The agency would do so only if large numbers of broadcasters begin to complain about interference, or they believe their livelihood is threatened “and they blame stuff on IBOC wrongly,” said one group engineering official.

Ray said the publication of the rules gives the IBOC technology certainty. “It takes the shroud off.” As for the FCC’s potential to change its nighttime IBOC rules, he said, “Could they come back and change some things? Of course they could; they’re the FCC.”

But he and most engineers contacted for this article said drastic interference would have to occur for the commission to rescind nighttime permission. The agency said its rules it would consider interference complaints on a case-by-case basis.

Several engineers believe that in cases of proven AM nighttime IBOC interference, FCC-imposed resolutions might involve requiring a station to reduce its HD-R injection level or requiring a station to reduce power in one digital sideband. The agency could also forbid certain stations from using IBOC at night.

Clark had an additional solution: Stations might agree between themselves to go directional at night and to face their signals away from each other.

Several engineers agreed the interference issue would eventually resolve itself as new IBOC receivers penetrate the market. But the question is what to do about today’s analog receivers.

Clark said many clients are angry it took so long to get approval for AM nighttime IBOC, and some have moved on to what they perceive as other revenue-producing ventures.

Time will reveal whether IBOC will help struggling AMs, he said. Getting in-dash, factory-installed HD Radios is vital because “hardcore early adopters,” such as those who buy aftermarket radios, “don’t make the Arbitron book,” he said.

“Radio today is run by accountants. The comptroller will ask, ‘What’s the payback?’ He doesn’t want to know the signal is going out farther. He wants to know how [IBOC] will translate into sales revenues and higher Arbitron ratings,” said Clark. He predicts that when a station manager perceives that the competition is beating his station, only then “will the purse strings open up for broadbanding.”

“You’ve got to have receivers in the dashboard for that to be a commercial force,” said Clark. Automakers are installing iPod adapters in the dash faster than they are HD Radios, he observed.