Re: [IRCA] [NRC-AM] Re: DRM vs IBOC

[Scott Fybush <scott@xxxxxxxxxx>]

Patrick Martin wrote:
> Scott,
> 
> Even though I have not detected IBOC Hash from any FM's I can hear on
> the coast from Portland & Seattle, I am told the adjacent is just as bad
> as on AM in metro areas. Do you find that true?

Thus far, no. My best test case here is the range from 102.1-102.9, 
where I have a semi-local on every channel:

102.1 - Albion, class A, 35 mi NW
102.3 - Canandaigua, class A, IBOC, 30 mi SE
102.5 - Buffalo, superpower B (110 kw), IBOC, 65 mi W
102.7 - Webster, class A, IBOC, 12 mi NW
102.9 - Syracuse, class B, 70 mi E

102.1 and 102.7 were late drop-ins, signing on in the 90s, while the 
others have been around much longer. When 102.7 first came on the air, 
its analog sideband splatter turned 102.5 from a reliable near-local 
into a bit of a DX catch. When 102.7 added IBOC, 102.5 went away on most 
car radios around town. But with the rooftop Yagi, I have no trouble 
separating out each of these stations. There's too much 102.7 signal to 
pull any digital audio out of the 102.5 IBOC, but I can get its analog 
just fine through the 102.7 IBOC sidebands. 102.1 is still fine on most 
car radios in the city, despite 102.3's IBOC. There's also no effect, at 
least that I've heard, of the IBOC carriers on the analog FM signal.

Except in a few exceptionally tight allocation situations (98.9 
Philadelphia/99.1 Zarephath NJ, both class Bs about 40 miles apart), the 
existing FM spacings around the country (on the commercial band, at 
least) are such that you have to look long and hard to find places where 
anyone other than a DXer would be listening to a channel first-adjacent 
to a local strong enough to create IBOC hash problems.

And of course FM doesn't have skywave to contend with. While there may 
be a stray summertime E-skip or trop opening that introduces some 
unwanted distant hash to a market far away, you'd never have a 
WTWP-WLAC-WWKB-WSAI mash of skywave hash on FM, as would be possible on 
AM. And unlike the AM system, there's demonstrated evidence from several 
DXers that the FM system (like DTV) can be DXed under the right 
circumstances. (I don't think anybody's pulled it off with meteor 
scatter just yet!)

No, it's still not perfect - there's an unanswered question, for 
instance, of what happens in Los Angeles if the class A on 102.3 in 
Compton lights up IBOC right over the sidebands of the Mount Wilson Bs 
on 101.9 and 102.7 - but except in the tightest spacing situations like 
that, the FM system basically works.

It should be noted, too, that engineers are still feeling their way 
around the FM system, too. There are a number of different ways to 
configure an FM IBOC transmitter installation, and some evidence to show 
that some of those configurations produce more unwanted sideband noise 
than others. This, too, will become a learning experience as engineers 
work out the kinks.
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