Re: [IRCA] IBOC vulnerabilities

[Barry McLarnon <bdm@xxxxxxxxxx>]

On Wednesday 19 September 2007 08:16, Craig Healy wrote:
> With IBOC turned loose, I'm going to take a different tack on some of this
> and focus on the actual bandwidth.  There is a requirement that stations'
> signals fit within a clearly defined mask.  I wonder how many are truly in
> compliance 24/7/365?  If the antenna system changes such as with wet
> ground, that may alter things.  Further, I would be willing to bet that
> many just barely fit the mask - or may not even be in complete compliance.
>  If that is the case, then if a station is found to be interfering, the
> victim may have an administrative recourse.  I would hope that Bob Savage
> considers this.
>
> Next week one of my clients is to have it's yearly RF occupied bandwidth
> and spurious signal tests done.  I'll query the fellow who does this about
> what he's found, and just what equipment and methods are required to make a
> case against an offender.  This house of cards may have a rather
> well-hidden vulnerability.  This needs to be examined more closely.

The method of checking mask compliance has been spelled out (by iBiquity, and 
rubberstamped by the FCC).  Some stations are probably out of tolerance, but 
even if they're forced into compliance, that won't solve most of the 
interference problems, which are inherent in the system design.

If the FCC was actually serious about restricting occupied bandwidth, they 
would have stopped IBOC in its tracks.  In my comments to the FCC on the IBOC 
docket, I described measurements in which I estimated that adding the IBOC 
digital carriers to an AM signal increased the occupied bandwidth of that 
signal by a factor of, believe it or not, 25.  25!!  Moreover, adding IBOC 
increases the average interference power dumped into a first-adjacent by 
anywhere from 18 to 39 dB.  In contrast, FM IBOC merely doubles the occupied 
bandwidth of an FM signal.  If you're interested, see 
http://topazdesigns.com/iboc/McLarnon-Comments-Jul05.pdf for a summary, and
http://topazdesigns.com/iboc/McLarnon-ReplyComments-Jul04.pdf for more info on 
the derivation.  Nobody from the IBOC camp has ever bothered to try to refute 
these numbers, since the adoption of IBOC was a political decision, not a 
technical one.  They do explain, however, why we have a mess today.

Of course, the Commission completely ignored these comments when it issued its 
Report and Order, since the fix was in.

Barry

-- 
Barry McLarnon  VE3JF  Ottawa, ON

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