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All-Digital AM? Check the Math

Engineer Jerry Arnold questions the notion

The author is a recently retired broadcast engineer in Terre Haute, Ind.

Credit: iStockphoto/art12321 Regarding the article “All-Digital AM Signal Called ‘Solid’ in Testing” (May 8):

When IBOC was just a fancy laboratory test, I stated that I saw a day when the digital-at-all-cost boys would make an attempt to force this digi-disaster on AM broadcasters.

They were successful in getting the FCC to allow broadcasters to use IBOC voluntarily at night. We saw what an utter failure it was, due to skywave interference and other reasons, so much so that many large-city AM powerhouses refused to use it at night or abandoned it altogether, such as Chicago’s WLS did.

Instead of admitting it was a failure, what did the folks at iBiquity and their financial partners CBS do? They claimed it was due to “not enough injection.” This, after assuring us that this was the fix-all to every problem AM broadcasters had!

One letter went as far as scolding those of us who had solid engineering reasons why IBOC wouldn’t work, especially at night, that “any AMer who had a 100-watt nighttime authorization ought to be all over IBOC.”

I made an editorial reply to this statement, using (dare I say?) engineering mathematics to prove my point using the writer’s own 100 watts of carrier power. Due to the “sort of” forward error correction format that IBOC employs from the lower to the upper sideband, the amount of IBOC transmitted with their then-standard injection amounted to a few milliwatts, which even in a laboratory environment would not give reliable coverage more than a few hundred yards from the antenna.

So now I learn that the boys are giddy about allegedly having a “solid” all-digital signal from a 10 kW transmitter on daytime at a distance of 13 miles — approximately 21 kilometers. Maybe I am again missing something, but assuming a quarter-wave antenna over average soil, 1 kW would produce a field intensity of 305.768 mV/m at 1 kilometer — 10 kW (the power given in the article) would therefore produce in the same antenna a field intensity of 966.023 mV/m.

I took the liberty of rounding the 20.9 kilometers (13 miles) referenced in the article to 20 kilometers, about a 4 percent change in their favor.

Using a frequency of 1 MHz, and assuming a good ground conductivity of 6 mS/m over the 20 kilometers, the resultant field intensity would be a robust 19.338 mV/m.

Why all the math?

Well, even the poorest quality Radio Shack transistor radio you can buy will clearly receive a 2 mV/m signal. What all this means is the digi boys are once again whooping it up for a modulation scheme that still requires 19.7 dB more signal to provide intelligible audio at the same distance.

And this with a 100 percent digital signal — no 5 kHz AM bandwidth left to “blame” for IBOC’s dismal showing. To put it another way: Without changing AM power levels, each station forced to go to an all-digital modulation will substantially lose coverage.

Was I prophetic when I predicted that there would be some who would begin to demand the FCC mandate this? Perhaps. Perhaps not. All I know is that the term “AM sunset” is particularly applicable if an all-digital modulation is mandated.

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