Re: [IRCA] IBOC DXing

["Rene' F. Tetro" <rtetro@xxxxxxxxx>]

One of the things I have wondered about with nighttime IBOC is what the
effects of fading, and other similar atmospheric phenomena, will have on the
IBOC signal.  Will the distortion of the IBOC signal caused by such things
effectively limit nighttime coverage?  With high power and medium power AMs,
what about areas where skywave and groundwave tend to overlap and cause
canceling or phasing effects? And, of course, the old favorite question,
what about nighttime skywave interference from adjacent IBOC stations within
a station's protected contour?  Suppose 880 WCBS in New York turns on IBOC,
and so does 890 WLS in Chicago.  These stations have strong enough skywave
signals within the other's "protected groundwave contour" that severe
interference is likely, if not guaranteed.

IBOC proponents are very big on telling us (DXers and others) that you have
no "inherent right" to listen to stations outside of their interference free
contour.  OK, there's nothing new there.  This has always been true -- you
get what you get outside of that contour, even with analog.  That simple
understanding is, in fact, the basis of our hobby.  What can we receive that
we shouldn't normally be hearing?  Isn't that the purpose of DX?  So, what
the proponents say on "protected contours" is really old hat.

BUT, I would suggest that what is really going to do in IBOC is when
stations start losing nighttime coverage within their protected contours due
to the 8-15khz sideband activity of the IBOC signal from distant stations.

Bottom line:  as has been said before, "The emperor has no clothes."  IBOC
(or more appropriately, IBAC) is bad technology that, in so far as nighttime
AM is concerned, will only work well in the lab, or in extremely controlled
circumstances.  In the real world of 10khz spacing and nighttime skywave,
it's a no brainer.  It's will, I predict, never take hold and will meet the
same fate as AM stereo.

73,
Rene'
Rene' Tetro
Lansdale, PA , USA
N2GQL, WPXG816, WPXU288, PG-2-16913
Coordinates:  40D12'32"N  75D18'23"W
Grid:  FN20IF
Email:  rtetro@xxxxxxxxx
Website Moderator:  www.radioveronica.us and www.dxhub.com

-----Original Message-----
From: irca-bounces@xxxxxxxxxxxxxxxx
[mailto:irca-bounces@xxxxxxxxxxxxxxxx]On Behalf Of Scott Fybush
Sent: Sunday, September 18, 2005 17:14
To: Mailing list for the International Radio Club of America
Subject: Re: [IRCA] IBOC DXing


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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