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AM IBOC Tries to Get Past the Noise

Will Interference Problems Derail AM's Digital Upgrade?

Will Interference Problems Derail AM’s Digital Upgrade?

As we contemplate what’s going to happen to the venerable AM broadcast band during the transition into hybrid IBOC operations, a myriad of unanswered questions remain.

We’ve seen FM IBOC adoption and implementation accelerate nicely, in spite of the delayed rollout of receivers.

Multicasting with SPS has suddenly ignited a new level of excitement throughout the industry. FM IBOC now has a real “killer app” to counter the increasing competition from satellite, iPods and wireless streaming devices.

Sadly, AM has no such weapon in its digital arsenal.


While AM IBOC still holds great promise for its ability to deliver dramatic improvement over its analog host with 15 kHz stereo reception, it only appears to be able to do that reliably inside the protected primary and NIF coverage contours. Even then there are problems with analog degradation on various models of existing receivers.

A number of stations are finding it difficult to suppress the “bacon-frying” noise produced in all analog receivers because of nonlinear performing transmission chains, including the null regions of many directional antennas. The necessity of having to rebuild antenna systems will place an expensive burden on numerous stations, many of which are the least able to afford it.

Using IBOC at night undoubtedly will unleash an ugly Pandora’s Box of trouble for many stations. The Canadian government has formally objected to U.S. stations operating IBOC at night, fearing significant interference to Canadian stations operating on adjacent channels. Neither the NAB nor the FCC had yet to comment on this objection at this writing.

We’ve suggested that for AM IBOC to have a better chance at succeeding, especially at night, the band needs to be thinned out.

Our proposal could offer an equitable way of reducing the number of AM stations by half and would certainly help reduce overall congestion and interference issues significantly. But even if they were to be adopted, the problem of adjacent-channel interference at night for many surviving stations would remain real.

Most AM station owners understand the risks of investing in the Ibiquity standard and are postponing plans to add IBOC until more of the lingering questions receive answers.

According to one resource as of this writing, there are 77 stations on the air with daytime AM IBOC. At least five Class A 50 kW blowtorches and a few other stations have turned it off, mostly because of interference caused to their own analog listeners or to their adjacent-channel neighbors.

Curiously, only 26 of the 77 stations feature music formats that would benefit the most by using IBOC digital while the rest are all news, talk or sports. The research for this is available online at: The information contained in this list appears to be valid and is, of course, constantly changing for a variety of reasons.


I’ve been a fan and supporter of the evolution of digital radio and the extraordinary effort of all of those who have helped develop the standard that now awaits adoption at the FCC, all the way from Project Acorn through the contributions of USADR, DRE, Lucent and Ibiquity.

I was an early supporter of IBOC for both AM and FM, but after carefully reflecting on how the AM rollout has faltered I am now convinced the proposed standard for AM during the hybrid transition period is not the best it could be. I am joined by many other engineers who see the possibility of a colossal train wreck coming, when and if AM IBOC is opened up for full-time operations by all stations.

Ibiquity has held firm to its original modulation design scheme for AM IBOC and has pushed it through on the back of the FM system. Receiver and chip-set manufacturers have largely committed to it as the only solution available to propel U.S. terrestrial broadcasting into the future.

But there has always been one fundamental problem with AM IBOC. The digital sidebands produce high levels of analog noise interference on both first-adjacent channels, resulting in degraded and even destroyed reception of the secondary coverage contours of many stations. Several studies indicate that 5 to 10 percent of all stations may experience this interference inside their protected primary contours.

Ibiquity has suggested that the laws of physics prevent any real resolution to this problem, and AM stations will essentially have to forget about their secondary and skywave service areas and accept the notion they are not important. Only the primary contour matters and will continue to be protected.

That’s been a hard pill for many AM owners and operators to swallow as they hunker down for the hybrid period with no definite end point for transition to a complete digital system.

Listeners who lose the ability to hear a desired and dependable radio service they’ve enjoyed for a long time could care less where a protected contour ends or begins. It will simply eliminate one of their favored choices and further diminish the size of AM radio’s already dwindling audience.

Why would we want to accept such a self-inflicted wound when it could be ameliorated by using better technology?


Other technology options that could make the digital transition easier are available.

Some involve smarter processing and filtering using DSP at the receiver end. On the transmission side, Digital Radio Mondiale offers some compelling options. DRM and the Ibiquity OFDM modulation structures are similar in many ways. Even though it does not showcase or promote its hybrid option that includes the existing analog component, DRM offers the advantages of a more flexible and scalable digital architecture.

The design is an existing world standard for digital transmission under 30 MHz. It is open and nonproprietary. Many parameters are dynamically adjustable on the fly including guard intervals and error correction. Channel bandwidths are adaptable as propagation conditions change, which offers a huge benefit for skywave listening. Most important, the DRM single sideband option could mean dramatically reduced interference for many first-adjacent stations at night.

Take the most notable and often used example of three Class A stations: WLW 700 in Cincinnati, WOR 710 in NYC and WGN 720 in Chicago. All three are 50 kW powerhouses that enjoy extensive secondary contour as well as skywave coverage and audience. Large areas of that will be lost in both analog and digital reception when all three light up IBOC at night.

But with DRM SSB, WLW could switch to LSB at night, while both WOR and WGN switch to USB, thereby reducing significant amounts of destructive interference. This case involves protecting useful service in secondary contours.

There are other examples too numerous to mention where the appropriate use of DRM SSB could reduce nighttime interference to protected NIF primary contours for many key stations and their adjacent-channel neighbors. Obviously the entire inventory of AM stations would need to be studied carefully to reveal how well the DRM option could provide significant reclamation of lost coverage, but my hunch is that it may be impressive.

Assigning appropriate DRM sidebands to every station that chose to operate digital at night would be challenging but could be done on an equitable basis. The FCC would employ the established precedent of longevity on the channel as used in determining nighttime skywave protection limits.


Unfortunately Ibiquity seems to have ignored DRM, mostly because of internal politics and its insistence that the business model and the standard they have constructed remain unchanged. It would appear that it’s just too unappealing to integrate other people’s software into a closed and proprietary structure that is poised to become a long-term for-profit monopoly.

But such a bold move could very well produce a better system, even though it may delay finalizing standards and chip-set designs in the short term.

Ibiquity may be facing potential litigation at the hands of Microsoft and others for keeping its HDC codec under wraps and out of the proposed FCC IBOC digital standard. Clearly the interests of everyone except Ibiquity would be better served if codec specifications could be part of the standard and left open to enable others to contribute future improvements.

That discussion is best reserved for another time. As FM IBOC deployment continues to gather momentum, Ibiquity insists on playing hardball in the face of rising opposition to several key parts of its blueprint for the future of terrestrial radio. It’s the sheer arrogance of Ibiquity that disappoints so many of us in the industry.


If the commission chooses to adopt the Ibiquity IBOC standard for unlimited AM day and night operations as proposed, here is a worst-case scenario that might unfold: A messy outbreak of interference complaints will be filed by adjacent-channel stations that lose existing analog coverage as increasing numbers of stations deploy IBOC full-time. Many of the complaints will be dismissed on the basis that the interference occurs in unprotected coverage areas, which is understandable and expected.

In cases where the interference complaints are legitimate, the offending stations will be forced to reduce IBOC power. In many cases, they may have to reduce it to a level where it’s not justified to operate it at all without leaving its listeners a poor impression of the new technology and the station’s inability to use it effectively. Would such stations be content to operate IBOC only during the day, outside of critical hours and nights?

The overall result of this chaos could leave AM IBOC an under-achieving digital standard that only a minority of stations could use to full advantage. We could see the filing of lawsuits by stations that invested in the technology but were forced to stop using it.

To resolve such disputes with an easy way out, the commission might allow such stations to negotiate “buyoffs” for these complaints to be able to keep using IBOC full time. The station that received the interference may decide that a cash settlement is more valuable than the “lost coverage.” The precedent of buying off your interference has already been, at least partially, established.

Many of the 5 to10 percent of stations that receive IBOC interference inside their protected contours may lose enough of their critical hours and nighttime coverage rendering operations during those times unprofitable and essentially useless.

Many of the rimshot stations that rely heavily on secondary coverage to serve their intended target audiences will probably be shut down by IBOC interference with no remedy whatsoever. Some may just throw up the white flag and be content to be daytimers or go out of business entirely. Others could get angry enough to pursue litigation.


The ensuing malaise would cripple AM further until such time that a significant number of stations simply go away and turn in their licenses. That may provide enough relief for at least some of the survivors to be able to use IBOC at night.

This might just be the FCC’s secret solution to curing congestion on the AM band. Eventually the commission would likely set a date for mandatory conversion by all remaining stations to full digital operations, leaving analog behind forever and marking a merciful end to the misery of the hybrid period.

Ibiquity would have you believe that such a worst-case scenario will never happen and that the hybrid conversion period will proceed with only a limited number of valid interference complaints. The affected stations will be able to negotiate mutually acceptable resolution with few complications or the need for many interfering stations to abandon IBOC operations completely at night.

History will probably record the actual course of events occurring somewhere between these two extremes.

It’s probably too late to hope or expect that Ibiquity might embrace the idea of changing its AM IBOC design to make it better. Or it may be too early, depending on your point of view. The introduction and eventual widespread use of smart radios that can receive software changes and upgrades seamlessly over the air will make future improvements so much easier.

It is entirely probable that this will happen during the hybrid period, making it more palatable for Ibiquity to consider and actually implement system modifications like DRM. Only time will reveal how the hybrid transition will fare.

RW welcomes other points of view. Comment on this or any story in Radio World Engineering Extra by writing to [email protected].