All-Digital AM? Check the Math
The author is a recently
retired broadcast engineer in Terre Haute, Ind.
Regarding the article
“All-Digital AM Signal Called
‘Solid’ in Testing” (May 8):
| Credit: iStockphoto/art12321
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
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
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
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
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