NRSC SAYS THERE STILL ARE TOO MANY QUESTIONS ABOUT AM IBOC
LAS VEGAS -- There are still too many questions about AM version of in-band, on-channel (IBOC) digital audio broadcasting (DAB) system to allow night-time operation, National Radio Systems Committee (NRSC) decided here Sat. NRSC earlier had approved full operation of FM version of IBOC, but committee members felt “the verdict is still out” on AM version, said Milford Smith of Greater Media, chmn. Of NRSC DAB committee.
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Evaluations of AM IBOC “compare very favorably” with analog FM in subjective analyses, said David Layer, NAB engineer who was committee member. However, he said, there remain concerns about skywave propagation at night and spectral occupancy. Key problem, Smith said, is that “there is a very difficult propagation mode at night” for AM and group needs more data to be sure about interference issues.
Skywave situation is “ugly” for all AM services, Smith said, and interference data at night “can change radically… There is just some data that it would have been very nice to have,” particularly about interference to first adjacent channel: “This needs to be studied further.” Among other things, Layer said, AM IBOC begins switching to analog backup mode at 14 millivolts of received signal strength at night, vs. 1 millivolt daytime, significantly reducing nighttime IBOC service area.
Test results did support approving AM IBOC for daytime operations, NRSC said. Layer said results, which are just being made available publicly on NRSC Website, demonstrate that subjective test results show “vast improvement” over analog AM in all types of programming. He said NRSC probably would file its full AM IBOC evaluation with FCC next week and expected Commission would seek comment.
Meanwhile, consumer electronics companies showed prototype consumer FM IBOC receiver at CES show in Jan., and expect to begin selling consumer units by next Jan., Layer said: “This thing is for real now. It looks like it is really going to happen.” FM IBOC caused only 0-0.54% increase in interference in most cities tested, said Alan Rosner of Denny & Assoc. He said most of impact was on fringe of station’s service area, where percentage of listeners was lower anyway, so figure might overstate number of people affected.
IBOC also will provide new datacasting opportunities for radio stations, said David Maxson of Bcst. Signal Lab. He said key would be to combine all data services into one data stream in order to maximize capacity, unlike wasteful subcarrier system used for analog radio. Paul Signorelli of Impulse Radio said there was “huge potential” for IBOC data services, which could carry 181 kbps data rate in addition to main audio signal.