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Which Stations Can Boost IBOC Beyond 6 dB?
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Now that the FCC has said it will allow most U.S. FM digital radio stations to increase digital power by 6 dB, what’s the mechanism for determining if a station can go beyond that four-fold increase, maybe up to 10 times the current digital power limit?
Stations will need to submit an application to the Media Bureau to increase FM digital ERP beyond 6 dB. The commission adopted a formula suggested by NPR and endorsed by iBiquity that will be used to help avoid harmful interference. Stations that want to raise their FM digital power beyond the blanket approval will have to calculate the station’s analog F(50,10) field strength at all points on the protected 60 dBu F(50,50) contour of a potentially affected first-adjacent channel analog FM station. For details read items 19 and 20 in the order. (So-called “Super-powered” FMs are treated differently in the ruling.)
In its order, the bureau stated that its experience with higher-powered digital experimental authorizations suggests that the NPR formula is “over-predictive of the potential for interference.” But the bureau believes “that the protection this methodology provides to first-adjacent channel stations can be used to establish an expeditious ‘go-no go’ mechanism.”
It remains to be seen how many stations will raise their digital power or qualify to go higher than 6 dB.
Engineering sources who have stations airing a digital signal have told Radio World they plan to implement an increase as soon as possible where they can. As RW has previously reported, some non-commercial stations — such as WAMU(FM), Washington and KUVO(FM), Denver — purchased digital transmitters with enough headroom to accomodate an FM IBOC power increase.
One indicator comes from a survey of non-commercial managers last fall; 70% told NPR that if a power increase were approved, they would raise their digital power within a year.
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