[BC] Re:Real cases of IBOC interference wanted for research purposes

[Phil Alexander]

On 20 Jul 2006 at 8:43, Tom Taggart wrote:

> I can see another -and more ominous turn- in the IBOC debate. 
> Looks like we have more consultants going over to the "dark side."

I believe Roy is only stating his understanding of the facts. The
actual facts of these two cases are somewhat more ominous and tend
to imply IMHO pattern bandwidth issues, about which I've written
Roy privately.
 
> Yes, in many cases the interference protection betweent two 
> adjacent AM stations is 1 to 1, that is, .5 to .5 . And for 
> many stations, there is quite adequate reception at this contour 
> daytime, with perhaps some minimal "monkey-chatter" from the
> other station.

However, the facts of physics are unchanged. The original IX rules
were predicated on a 50:1 D/U ratio (co-channel) for interference
free reception, dropping to about 10:1 at the second adjacent. To
satisfy demands to allow the have-nots to put stations on the air,
the restrictions flowing from these facts were degraded over time
to the point that 50:1 has dropped to 20:1 and 2nd adjacent has
dropped from 10:1 to a mere 2:1 (partly as a result of poor receiver
response and use of the NRSC mask restricting transmission bandwidth. 
This is the result of political influence, overt and covert, at the 
Commission, but it does not change the underlying physics.

> Pre-IBOC
> 
> The example I site of WLJM Lima and WWJ Detroit we actually have 
> WWJ's .25 just approaching WLJM's .5 in Hancock County, Ohio 
> (Findlay). Hence this is a 2 to 1 signal for WLJM. Without IBOC 
> this produces a listenable signal at the WLJM .5, for WWJ's
> 940 signal is barely noticable.
> 
> The WWJ IBOC signal places destructive interference on the 
> adjacent channel.  It matters not whether this is "inside the 
> mask" or not. The operation of IBOC means that .5 is no longer 
> a "protected contour," instead some higher value of signal is 
> required before one can reach the same level of "interference 
> free" signal one had with the .5 contour before IBOC.

One might argue that physics shows it has not been protected for
many years, although it is true the IBOC IX is much more 
objectionable to the average listener than typical "monkey
chatter."

> This is a significant change in the protected coverage of an AM 
> signal. 

This is true only if you take subjective factors like the grating
character if IBOC IX into account. Objectively, the theoretical
IX is not significantly different.

> It should not be done by the Commission by a blanket grant of 
> approval for IBOC without case by case consideration of existing 
> station's coverage.

What? You want another full employment act for the entire FCC bar
and all who aspire to it? <g> Realistically, Tom, you know and I
know it ain't gonna happen, not with the post-1960's Commish it
won't. The only practical way is turning all the IBOC's on who
want and can afford it, and then deal with the ones that cause
IX on a case by case basis, although communications bar full
employment will result IMHO either way. Physics is what it is,
and fortunately its laws are beyond human amendment and tampering.


---------------------------------------------
Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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