NPR Tests Radio Interference on Channel 6
October 3, 2008
It's been 28 years since comprehensive tests were performed the establish interference level thresholds from noncommercial and educational FM radio stations (88.1-91.2 MHz) to adjacent television Channel 6 (82-88 MHz).
With the age of digital television upon us, and since some TV broadcasters have elected to transmit their DTV signals on Channel 6 after the February 2009 transition, National Public Radio began a series of tests in November 2007 on the FM interference potential to new consumer DTVs. The results are in and last month, NPR informed the FCC that the lab results show "significant improvement in interference immunity."
The testing involved 17 digital TV receiving devices selected for the NPR testing lab in Washington, D.C., by the consulting firm of Meintel, Sgrignoli and Wallace. They included low-end set top box converters, personal computer tuners, table-model TV sets and also flat screen models. These units were representative of fifth-generation tuners.
Testing methodology consisted of applying a variable level FM broadcast signal to the antenna terminals of an operating DTV receiver and observing the level at which picture degradation began. Tests were done using every FM channel from 88.1 MHz to 91.9 MHz (Channels 201 to 220). Laboratory digital TV signals were generated with a Harris Broadcast CD-1A exciter driven with a sports broadcast recording to provide moving images. The interfering FM signals also included an IBOC component.
Testing was performed using two different RF DTV channel six levels to simulate medium and high received signal strengths (-53 dBm and -28 dBm respectively). Median receiver data showed FM interference thresholds from 20 to 50 dB above the DTV signal (at -53 dBm) and from 27 dB to 43 dB at the higher DTV signal level injection. The DTV signal was backed off to -68 dBm, with the result that the threshold of visibility from the interfering FM source began at 20 dB on the lowest FM frequencies, and lowed to 10 dB on the high end frequencies. The report summary notes that actual ratios approached 50 dB above the channel six signal, "where ratios were limited by noise within the RF test bed."
The complete report summary is available here.