Re: [IRCA] IBOC DXing

[Scott Fybush <scott@xxxxxxxxxx>]

At 06:08 PM 9/18/2005 -0400, you wrote:

>That could be true for an individual station that itself is not
>suffering from sidebend IBOC interference.  The real test would be
>how would stations fair in an environment crowded with IBOC signals.
>Mr'. Ray's testing notwithstanding, there has not been enough testing
>to determine the extent of this problem.
>
>Bill Harms

This is absolutely true, and it points out an issue that smart AM engineers 
have been working with for years. The usability of an AM signal for the 
average listener (we, as DXers, are clearly the exception) is based on two 
factors.

The obvious one is the level of signal strength received from the desired 
station. I'll save David the typing, since he's made the point often 
enough, but it's generally agreed that an average casual listener needs at 
least 10 mV/m, and often quite a bit more, to comfortably listen to an 
analog AM station on a garden-variety radio in today's noisy environment. 
In a big city, especially if you're dealing with lots of steel-framed 
buildings and many sources of electrical noise, 25 mV/m is highly 
desirable. (And indeed, the FCC once recognized that, requiring stations to 
put 25 mV/m over the main post office of their community of license. This 
was later relaxed to 5 mV/m over the entire community, making possible a 
number of substandard signals that would not have been allowed back in the 
day.)

The less obvious one - but one we, as DXers, should immediately recognize - 
is the level of received interference. The engineering guy I know at one of 
the biggest groups out there tells me that when they're looking at a 
potential AM acquisition, there's only one number that really matters to 
them. It's not raw power, it's "NIF" - the station's nighttime interference 
free signal contour, which is a measurement that's calculated by summing 
the incoming signal level from the OTHER stations on the same and 
co-channel that contribute the most interference to the desired station. 
The lower the NIF, the less signal an AM station needs to put over an area 
to overcome the incoming interference, and thus the greater the area that 
station can cover after dark with a usable signal.

Therein lies the validity of the concerns of nighttime IBOC opponents: the 
addition of all those digital carriers on adjacent channels would 
dramatically increase the noise floor across the entire band.

The net effect would be to further broaden the gap between the "have" 
signals and the "have-not" signals. Tom Ray has nothing much to worry about 
at WOR; by day, his signal is already very well protected from interference 
not only on 710 but also on both first-adjacent channels. (I concur with 
Tom that in the real world, any effect WOR's outermost subcarriers might 
have on outlying signals on 690 and 730 is minimal, and affects those 
stations only in areas outside their normally protected contours.) He's 
safe at night, too - remember, some of the earliest testing that was done 
at night involved WOR and WLW, and the conclusion was that WLW's skywave 
within WOR's home market wasn't strong enough to interfere with digital WOR 
reception, and vice versa. (Again, this is within each station's home 
market; we know full well that IBOC operation on both signals wipes out 
analog on both of them in areas outside their home markets, where each 
station's signal level is roughly equal.)

But WOR is one of the best-protected signals in the country. Decades of 
poor decision-making by the FCC have created too many situations where 
class B, C and D AM signals are packed far too close together for optimum 
analog performance, and even Ibiquity itself acknowledges that there are at 
least a few circumstances where nighttime IBOC won't be possible. Even by 
day, we've seen some examples (WXYT 1270 Detroit against WLBY 1290 Saline 
MI, 1150 against 1170 near Denver) of measurable incursion by IBOC 
subcarriers into the protected contours of a neighboring analog signal. 
It's those stations, not the WORs of the world, that will feel the bite 
from IBOC.

What's really happening here, though most broadcasters haven't quite 
figured it out yet, is that the approval of IBOC is bringing about a very 
real change in the coverage that AM stations can expect to enjoy. The 
stations that benefit from "bonus" coverage outside their protected 
contours face the loss of much of that coverage as the noise floor rises 
under them, and what's left of the old wide coverage of the "clear 
channels" at night will be severely limited by the mutual interference from 
adjacent stations' digital carriers.

Is that a fair tradeoff for the improved service that IBOC *may* allow AM 
stations to provide to their home markets? It may be, especially if you're 
a station like WOR that depends very little on attracting listeners beyond 
its core market...but it would certainly be nice if the FCC's deliberative 
process had been framed this way, rather than painting anyone who listens 
to distant signals as a "skywave enthusiast" (Ibiquity's term) worthy of no 
attention in the debate.

All of which is a very long way of saying that both Barry McLarnon and 
David Gleason are right, in their own ways. They're just approaching the 
issue from completely different frameworks, which is no good way to reach 
any kind of informed decision on whether or not the tradeoffs inherent to 
IBOC are worth it.

s

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