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Building at –14, Planning for –10

One engineer, an HD Radio believer, shares thoughts about the power hike

U.S. FM stations won the ability about three years ago to seek higher digital power injection levels. I’ve been working on a story about that for a pending Radio World “eBook” project. One source asked not to be identified but provided some interesting insights so I’ll share them here instead.

He’s a long-time believer in IBOC; and at his broadcast company, new FM RF installations are being built with an ultimate –10 dBc operation level in view, even though most are currently at –14 with a few exceptions sprinkled in. I asked him how he would characterize the general success of higher power so far, in general.

“The implementation has been slower than the FCC expected and about on pace with what I would have expected,” he told me. “When the power levels were first authorized, many stations could not participate because of the expense. Since that time Harris, Nautel and ERI have developed solutions that make high-power IBOC affordable.”

He says digital operation at 14 dB below carrier definitely brings digital coverage closer to the analog, and improves indoor reception, as its advocates have hoped. “The biggest improvement is decreased blending at the edge of coverage and indoor penetration.”

I wondered how much extra digital power is enough to make a real difference in digital radio performance. He replied, “With digital, even a 1 dB increase can make the difference between 100% high-quality reception and none. The iBiquity system as designed at –20 was predicted to deliver a similar coverage to the analog; however, the FCC and consulting firms had no information as to the prevailing noise levels in the FM band, and when the system was actually implemented, it was found that the noise levels were significantly higher than were used to predict coverage.”

For fixed reception, he said, 3, 2 or even 1 dB can make a difference, and for mobile 6 dB makes a big improvement.

“Ultimately,” he said, “we need –10 dBc as an across-the-board level except in limited number of situations.” But he’s not a fan of asymmetrical sidebands, calling this “a prophylactic consideration” to deal with errors in interference studies done by NPR Labs. In his view, –14 dBc is just an interim step.

I’ll have more about digital power levels in our latest Radio World eBook, to be published shortly. Meantime, share any comments at [email protected], or post a comment below.