Further Study of FM IBOC & FM SCA Compatibility
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May 24, 2002 Re: Digital
Audio Broadcasting Systems and Their Impact On the Terrestrial Radio Dear Madam Secretary, Pursuant to the Public Notice (DA 01-2932), dated December 19, 2001, in the abovereferenced matter, National Public Radio (NPR) and the International Association of Audio Information Services (IAAIS) respectfully submit the attached “Further Report on Analog SCA Compatibility with iBiquity’s Digital FM-IBOC System.” NPR, IAAIS and
the Advanced Television Technology Center jointly crafted the Using the study’s
data, NPR contracted V-Soft Communications to prepare a visual NPR and IAAIS
stand ready to assist the Commission in considering the technical
and policy implications of the study’s results in the referenced
proceeding.
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Executive
Summary
During 2001 the National Radio System Committee (NRSC) conducted tests
to evaluate
the compatibility of iBiquity Digital’s FM IBOC system with analog
subcarrier (SCA)
operations. National Public Radio (NPR), and the International Association
of Audio Information
Services (IAAIS) represent members who operate SCA analog facilities.
The IAAIS represents
over 100 radio reading services for the blind and print-disabled. NPR
provides programming and
representation to over 640 public radio stations, many of which operate
SCAs. Reading services
in thirteen of the top sixteen radio markets utilize subcarriers of NPR
member stations.
During NRSC deliberations, NPR and the IAAIS noted the inconclusiveness
of test
results concerning analog SCA compatibility with IBOC transmissions. The
parties suggested the
need for additional data points to achieve greater resolution and a better
understanding of the
likely interference consequences to analog SCAs of a mass deployment of
IBOC transmissions in
the FM band. Additional information was sought to analyze the effects
of IBOC transmission
on subcarriers operating on different frequencies (e.g. 92-kHz vs. 67-kHz)
and design variables
affecting the relative susceptibility or immunity of different SCA receivers.
iBiquity Digital volunteered
to fund further tests to help resolve these concerns and
sought to conduct the tests as expeditiously as possible for their consideration
by the FCC
during the current comment period on terrestrial digital radio. IAAIS
provided ten SCA receivers
believed to be representative of those currently in use. iBiquity, IAAIS,
NPR and Advanced
Television Technology Center (ATTC) staff, as well as outside consultants
to all parties,
collaborated in shaping test criteria and observing the tests that were
performed by the ATTC
during February and March 2002. Additionally, multipath issues were raised
as a significant
factor affecting SCA performance and additional work was done to analyze
the audible effects of
multipath on SCA performance. The ATTC’s documented test plan and
summarized test results
were included in iBiquity Digital’s filing before the FCC on this
matter dated 13 May 2002.
At the conclusion of testing,
NPR contracted with V-Soft Engineering of Cedar Falls,
Iowa to model the results in the top sixteen radio markets using Longley/Rice
propagation
predictions and incorporating predictions from the test data about SCA
analog receiver
performance. In addition to revealing areas and populations where terrain
effects would limit
reception, areas and populations were calculated and plotted where coverage
would be limited by
existing analog adjacent channel interference, as well as the areas and
populations calculated to
lose coverage due to new IBOC interference (assuming all 1st and 2nd adjacent
channel stations
were transmitting IBOC signals). The performance characteristics of the
SCA receivers were
fairly closely grouped, and an average of their performance was used in
generating the predictive
impact maps. The coverage studies were projected to extend to the 54-dBu
contour, based on an
assessment that few SCA receivers are in use beyond this contour without
the aid of outdoor
antennas or cable FM input. This assumption was tested for four markets
where actual SCA
receiver distribution was available by Zip Code. These maps are included
as Appendix A.
A tabular summary of the raw
population counts within the projected SCA reception
areas, the areas and populations delimited by existing analog interference,
and the new areas and
populations projected to be interfered with by the addition of FM-IBOC
is included as
Attachment B. Pie-charts representing these effects on the populations
of each of the markets
studied are included as Attachment C.
On average, the added IBOC-interference
effect, based on population, was projected at
2.6%. This figure varied considerably, from as low as 0.3% in markets
with relatively flat terrain
and high transmitter power and antenna height to 8.5% in the Los Angeles
market on station
KPCC. In general, lower power stations suffer greater interference, both
from adjacent analog
stations, as well as from new potential IBOC interference.
There appeared to be no discernible
distinction in the susceptibility of 92-kHz SCA
receivers versus 67-kHz receivers. Additionally, second adjacent channel
interference contributed
the bulk of interference attributable to IBOC operations. Apparently,
first adjacent channel
IBOC interference was effectively “masked” by the analog interferer,
whereas IBOC energy
added to second adjacent station’s signal contributed a measurable
increase in undesired signal
competing with the capture of the desired SCA signal.
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