NAB: 'LPFM Report Fatally Flawed'
This is a summary of the NAB's response to the recent Mitre
Corp. report to the FCC on the topic of low-power FM service. This
was published in the association's Radio TechCheck and is reprinted
here with permission.
Four years ago the Federal Communications Commission (FCC) initiated
a rulemaking proceeding for the creation of a low-power FM (LPFM)
radio service. According to the Commission's proposal, 3rd adjacent
channel interference protection for existing full-power FM stations
could be eliminated based on the assumption that listeners to these
full-power stations would not be adversely affected. The Commission
concluded that eliminating 3rd adjacent channel interference protection
for full-power stations would allow a significant number of LPFM
channels to be allocated nationwide.
NAB, the Consumer Electronics Association (CEA) and others objected
to this proposal, saying that listeners to full-power FM stations
could receive significant interference should low-power stations
be allowed under the FCC's proposed plan. The FCC was repeatedly
urged by commenters to conduct field tests in order to prove that
there would be no objectionable interference to full-power FM stations.
However, the Commission simply stated there was no valid reason
to conduct field tests and proceeded with its plan.
Congress became concerned about the FCC's rush to judgment and
the way the FCC was attempting to justify the removal of interference
protection for existing FM stations. Listeners should not be deprived
of their ability to receive free over-the-air broadcasts of their
choice including vital weather and other life-saving emergency information
historically provided by full-power stations. Congress therefore
directed the FCC to conduct audience listening tests to determine
what is objectionable and harmful interference to the average radio
listener.
By way of a lengthy selection process, Mitre Corporation was chosen
by the FCC to perform the tests mandated by Congress and publicly
report the results of those tests. On July 11 this year, the FCC
released a Public Notice seeking comment on Mitre's technical report
on the 3rd adjacent channel impact of LPFM stations. Mitre concluded
that any interference created by the elimination of existing 3rd
adjacent channel protection to full-power FM stations was insignificant.
NAB, with the assistance of the engineering consulting firm Carl
T. Jones Corporation, thoroughly reviewed the Mitre Report and concluded
that the report contains so many major technical errors and omissions
that the resultant test data is rendered unusable. NAB commented
to the Commission that based on this fatally flawed report, the
FCC cannot recommend to Congress the elimination of 3rd adjacent
channel protection in order to accommodate additional LPFM stations.
Here are a few of the major technical errors NAB found in the Mitre
Report:
1) Mitre's "sample of one" listening test was not scientifically
valid. During tests to determine if LPFM interference would be objectionable
to listeners of full-power stations, Mitre relied on only one person
to make this determination in a simple "yes" I hear interference,
or "no" I hear no interference. A scientifically valid
audio listening test is always conducted in a blind fashion such
that listeners are not aware of the details of any particular audio
sample. The Mitre listener always knew the details surrounding the
audio sample, therefore biasing the results of his observation.
Additionally, listeners in a scientific test are always properly
trained and typically report their observations on a multipoint
degradation scale. This was not done according to the Mitre report.
2) Mitre did not provide any data showing that it had characterized
the receivers being used in its test. It is a well-known fact from
previous scientific receiver studies that the IF passband of many-if
not most-receivers is not symmetrical. Mitre only tested the receivers'
susceptibility to upper 3rd adjacent channel interference. Studies
conducted by the CEA show that receivers can show as much as a 30
dB difference in sensitivity between upper and lower adjacent channel
interference sources, based on the relatively common non-symmetrical
receiver IF passband.
3) Mitre's tests were performed with receivers located inside a
vehicle with no information provided on how RF signals were coupled
into the receivers-except that each receiver had a separate antenna.
The spectrum analyzer used to determine a desired-to-undesired (D/U)
signal ratio also used its own antenna. A scientifically valid test
requires that careful consideration be given to the distribution
of signals to multiple receivers. Based on Mitre's test setup, it
is clear that the D/U ratio at the input to the spectrum analyzer
is known. However with each receiver utilizing a separate unknown
antenna, no determination can be made as to the actual D/U ratio
at the input to each receiver.
The NAB comments in this proceeding (MM Docket No. 99-25) contain
many more examples of the poor engineering practices used by Mitre
during these tests - tests which were commissioned to show that
listeners could continue to receive their favorite stations without
additional interference caused by LPFM stations. The complete text
of NAB's Comments to the FCC may be viewed and downloaded from the
NAB Web site at www.nab.org.
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