AM IBOC Tries to Get Past the Noise
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.
GREAT PROMISE, GREAT PROBLEMS
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:
http://topazdesigns.com/iboc/station-list.html. The information
contained in this list appears to be valid and is, of
course, constantly changing for a variety of reasons.
ROOM FOR IMPROVEMENT
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?
DRM TO THE RESCUE?
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.
PROTECTING THE FRANCHISE
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.
BACK TO THE FUTURE
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.
END-GAME STRATEGIES
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.
Guy Wire, Radio World's masked engineer, is the pseudonym of
a well-known radio veteran. Opinions are his own.
RW welcomes other points of view. Comment on this or any story
in Radio World Engineering Extra by writing to radioworld@imaspub.com.
| Sponsored links: |
|
Transradio: DRM, AM, VHF/FM - We make the transmitters. Visit us now at www.transradio.de for more information.
RF Central - Total RF solutions manufacturer (TV broadcast): Full-Service 2GHz Relocation, COFDM, HDTV ENG components, complete links.
Nucomm delivers industry-leading microwave solutions for high-data-rate HD and IP File transport applications from portable ENG/OB to rack-mounted fixed link systems. Click here!
QuStream's signal conversion and processing products set the signal standard using patented technology to convert, encode, decode, synchronize and process video signals. Click here!
MultiDyne provides a wide array of video and fiber optic transport solutions, each with the highest image quality in the industry. Click here!
Harris Corporation's Broadcast Communications Division designs products that streamline workflow of content production, processing, transmission, management, storage, test and measurement and broadcast graphics. Click here!
|
|
