Re: [IRCA] IBOC DXing

[Scott Fybush <scott@xxxxxxxxxx>]

At 02:56 PM 9/18/2005 -0400, you wrote:
>IBOC would require 500 kW of power to equal analog coverage on 50 kW. 
>There is simply no room for 500 kW AM stations in the U.S., with the AM 
>allocation table the way it is. If you remove the analog signal, then the 
>IBOC signal on 50 kW would get out, at most, the same range as a Class 
>B1/C3 FM station (25 miles). A 1 kW graveyarder, with the analog signal 
>removed, would cover less ground than a 10-watt LPFM.

We're all entitled to our feelings about IBOC, but let's try to stay 
somewhere close to reality when we're discussing its implications on our hobby.

I'm not sure where Eric's making up his "facts", but real-world experience, 
even to the limited extent we now have it, shows that groundwave coverage 
of MW IBOC digital subcarriers is, absent significant interference on the 
first-adjacent channels, roughly equivalent to analog coverage on the main 
carrier - and that's with the digital subcarriers operating at a fairly 
small fraction of the analog power. (For a 50 kW AM station, you'd be 
looking at about 1500 watts of digital power.)

Real-world example: I had the opportunity to visit fellow DX'er Kent 
Winrich in Milwaukee about a month ago, and we spent a little time out in 
his truck listening to his IBOC tuner. No fancy DX rigs here - just a 
fairly standard headend (Kenwood? Pioneer? I forget which) using the 
factory antenna in his SUV - and from some 90 miles out we had no trouble 
listening to the digital signal on WBBM-780.

We didn't have a field-strength meter with us (or at least, Kent didn't dig 
it out of the back of the truck if we did), but WBBM's calculated to 
deliver about 7 mV/m of signal on its analog carrier where we were 
listening. The digital carriers would have been down at least 3 dB from that.

Removing the analog signal would only HELP the situation, not hurt it, 
since it would allow the digital carriers to return to the protected core 
of each station's spectrum. Considering that WBBM, as a class A station, is 
protected by day to its 0.5 mV/m contour, that would give you something 
closer to well over 100 miles of solid coverage, conservatively, on a 
digital-only signal, and that assumes that you're still using the reduced 
power levels now in use for digital. Put the digital carriers on the full 
50 kW of WBBM's analog signal and you're looking at digital coverage that 
would easily exceed the already substantial range that BBM's analog signal 
enjoys across the good ground conductivity of the upper midwest.

The situation for graveyarders is more complex, because you're dealing with 
substantial received interference that isn't a factor on the nice clean 
WBBM signal, but even there, the error-correction intrinsic to IBOC (or any 
other competently-designed digital signal) should help a digital-only 
signal on a graveyard channel overcome some of the noise that plagues those 
channels now.

More real-world evidence comes from the testing so far of DRM, which is 
functionally equivalent, in terms of power requirements and propagation, to 
what a digital-only Ibiquity system would be like. All indications thus far 
are that long-distance reception of DRM signals on SW and MW can be 
accomplished with power levels equal to or even lower than current analog 
power levels. (Don Messer of DRM gave an interesting paper on some European 
MW tests at last spring's NAB convention, and it's from that talk that I'm 
drawing these conclusions.)

Oh, and of course there's also no such thing as an "AM allocation table," 
at least not for domestic purposes in the U.S.

We're all going to come face to face with the reality of IBOC soon enough, 
if we're not already. Spreading made-up "facts" only muddies the water 
unnecessarily.

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