NRSC Endorses FM IBOC
by Leslie Stimson
Ibiquity Digital Corp. has cleared an important hurdle
on the road to commercial rollout of its in-band, on-channel digital
audio broadcasting system.
The standards-setting body evaluating the viability
and practicality of implementing IBOC digital radio has studied
Ibiquity’s FM system and declared it "a significant improvement"
over analog.
The DAB Subcommittee of the National Radio Systems
Committee has recommended that the FCC approve the system for use
in the United States, specifically citing its "greatly reduced
impact of multipath interference (for mobile, portable and fixed
receivers alike); superior resistance to co-channel and adjacent
channel interference; support for enhanced data services and improved
audio quality."
In a more than 200-page document released in late
November, the NRSC achieves a long-stated goal. After nearly a decade
of encouraging IBOC development, the NRSC now states definitively
that IBOC has significantly enhanced performance over analog.
‘Remarkably robust’
"A lot of the data is quantifiable. We have
analog data and new digital data that can be compared handily,"
said NRSC DAB Subcommittee Chairman Milford Smith.
"The whole goal is to come up with something
that’s better than what we’ve got. This is a remarkably
robust system that provides a lot of advantages for the listener."
By approving the system, the committee states, the
FCC would be "charting the course for an efficient transition
to digital broadcasting with minimal impact on existing analog FM
operation and no new spectrum requirements."
Ibiquity President Robert Struble called the report
a validation of Ibiquity’s FM system. He hoped the FCC would
soon put the report out on Public Notice and seek public comment.
NAB President and CEO Eddie Fritts said the endorsement
is "a key step in moving broadcasters into the digital era."
Unclear immediately after the NRSC action was what
the FCC would do with the report. Several sources believed the commission
would seek public comment on the document as a first step, then
perhaps ask the NRSC to begin a formal standards-setting process.
The commission easily could craft a blanket authorization for stations
to implement IBOC, sources said.
Any of these scenarios would fit with Ibiquity’s
goal of having transmitter partners Harris, Broadcast Electronics
and Nautel show IBOC-compatible exciters and transmitters at NAB2002
in the spring.
Tradeoffs
However positive the NRSC is on IBOC, members were
careful to say there will be tradeoffs in terms of compatibility
between the digital signal and its impact on adjacent channels.
In the Ibiquity system, the digital sidebands of
the host station sit in the first adjacent channel of the next station.
The amount of digital energy being transmitted is relatively low,
around 23 dB, but Ibiquity told the NRSC when it submitted its test
results that field test results indicate there could be some impact
on analog first-adjacents (RW, Sept. 12.)
Smith and NAB Science and Technology Vice President
John Marino said the NRSC believes the tradeoffs are modest and
acceptable.
"Some of the earlier versions of IBOC systems
were horrific in terms of compatibility," said one source.
In its report, the NRSC wrote this about the implementation
tradeoffs:
"With respect to the main channel audio signal,
evaluation of test data shows that a small decrease in audio signal-to-noise
ratio will be evident to some listeners in localized areas where
first-adjacent stations, operating with the FM IBOC system, overlap
the coverage of a desired station.
"However," the committee continued, "listeners
in these particular areas may also be subject to adjacent channel
analog interference, which will tend to mask the IBOC-related
interference, most appropriately characterized as band-limited ‘white’
noise, rendering it inaudible under normal listening conditions.
"Also, all present-day mobile receivers include
a stereo blend-to-mono function dynamically active under conditions
of varying signal strength and adjacent channel interference. This
characteristic of mobile receivers will also tend to mask any IBOC-related
noise. The validity and effectiveness of these masking mechanisms
is apparent from the rigorous subjective evaluations performed on
the data obtained during the NRSC’s adjacent-channel testing."
The committee provided further findings about the
impact on non-host analog signals:
"Co-channel interference: no impact on analog
reception.
"First-adjacent channel interference: listeners
within the protected contour should not perceive an impact, but
a limited number of listeners may perceive an impact outside of
the protected contour under certain conditions.
"Second-adjacent channel interference: NRSC
tests indicated that some receivers (with performance similar to
the NRSC analog automotive and portable receivers) should not experience
an impact on performance due to second-adjacent channel hybrid FM
IBOC interference, however a very limited number of receivers (with
performance similar to the home hi-fi receiver used in the NRSC
tests) might experience a negative impact for -30 to -40 dB (and
more negative) D/U ratios."
Listeners should not perceive an impact on analog
host station reception in the FM hybrid mode, the NRSC states in
its report.
More data
The NRSC has yet to evaluate the system for unimpaired
audio quality; it will do so when Ibiquity delivers more test results
after integrating its Perceptual Audio Coding algorithm rather than
MPEG-2 AAC, its former audio codec.
Questions remain about the system’s effect on
analog FM subcarriers. Receiver results varied widely, sources said.
As a result, Ibiquity, National Public Radio and the International
Association of Audio Information Services have agreed to perform
additional tests to determine how certain SCA receivers will perform
after IBOC is implemented on host and adjacent channel stations.
Results would be given to the FCC directly, sources said.
Smith said this latest effort to evaluate IBOC was
the most comprehensive so far, from the sheer amount of data the
group studied.
The evaluation working group, a subset of the DAB
Subcommittee, reviewed lab and field test results of Ibiquity Digital
Corp.’s FM IBOC in eight performance areas: audio quality,
service area, durability, acquisition performance, auxiliary data
capacity, behavior as signal degrades, stereo separation, flexibility
and the IBOC signal’s impact on the host and neighboring stations.
The report is the result of efforts from several
volunteers, led by Dr. Don Messer of the International Broadcasting
Bureau.
The NRSC hoped to begin evaluating Ibiquity’s
AM system at its next meeting in mid-December. It hopes to have
that report completed by early March.
Also in November, the International Telecommunication
Union initially endorsed Ibiquity’s FM system for use abroad
and intended to put a draft recommendation for a vote before its
189-country membership.
In April, the ITU had endorsed Ibiquity’s AM
system (below 30 mHz) for use in other countries along with the
Digital Radio Mondiale AM system for long-wave, medium-wave and
shortwave use.
The full NRSC report is available on its portion
of NAB’s Web site, at www.nab.org/SciTech/Fmevalreportfinalfinal.pdf.
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