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.