[BC] Blending issue

[Mike McCarthy]

No, this is a real issue.  Not sure what to do yet.  I recall having heard 
about swapping the AES polarity. But does THAT invert the analog signal?

MM

At 01:53 PM 11/17/2005 -0600, Jeff Welton wrote
>Barry commented:
>
> > There has been some recent discussion about a phase problem with the
> > audio when IBOC receivers "blend" to analog.
> >
> > Apparently there are cases where the analog and the "decoded digital"
> > are out of phase to each other, and the radio goes "quiet" for a
> > moment.  Some have attributed this to a recent software upgrade on
> > some IBOC exciters.
> >
> > Another person told me this week that this issue has been known for
> > some time, and pointed to a specific processor.   I won't name it
> > here, because I am not in position yet to verify that.
> >
> > However, it may be useful for anyone running IBOC to verify the blend
> > is without "dropout."
>
>Barry - if I was the person to whom you're referring, I misled you 
>(sorry!).  I'm going to plead that I didn't completely understand the 
>question over the phone (bad Jeff, go to corner, write, "understand, then 
>respond" 100 times).
>
>While I haven't really been following the topic, the above sounds like 
>primarily an issue of synchronization.  Folks, it is CRITICAL that you 
>synchronize the analog and digital audio signals - even if (in the case of 
>FM) the analog audio does not go through the HD generator.  In the case of 
>FM, not synchronizing the audio can definitely cause dropouts when 
>blending between analog and digital.  In AM, lack of synch will also cause 
>dropouts and can, on occasion, cause the HD generator to lock up if the 
>two signals meander far enough out of phase.  Our NE-IBOC generators (I 
>assume the rest, but don't know for sure) have a couple of ways with which 
>you can synch the audio.  First, there's a sample rate converter provided 
>in the auxiliary unit that is part of the NE-IBOC package.  This can be 
>connected to the output of the STL (or whatever receives the program audio 
>at the transmitter site) and used to drive both the analog and digital 
>processors.  Alternately, the NE-IBOC main unit has three 10MHz clock 
>outputs which can be used to synchronize external devices, or one of the 
>sample rate converter outputs could also be used to synchronize the 
>processors, if the processor has the ability to synch to an external 
>input.  There are other options, don't hesitate to call your manufacturer 
>for additional information
>
> > ... that is ... IF you have a receiver.  Amazingly, I still hear of
> > stations installing digital without even having one receiver on hand.
>
>I've been to a few where the only digital receiver was in the GM's car - 
>not available at installation time - or in the regional DOE's car - also 
>not available.  I've only installed two that I can think of where we 
>actually had a receiver done up so that we could listen in the site to 
>what we were putting on the air.
>
>Just some thoughts from the rainy, foggy North East (I'm not complaining, 
>a customer in Tennessee just told me it was 20 degrees there (near 
>Knoxville) this morning - the lowest temperature we've seen so far 
>recently has been about 40).
>
>Best,
>
>Jeff Welton
>Customer Service Dept.
>Nautel Limited
>Tel: +1-902-823-3900, ext. 127
>Fax: +1-902-823-3183
>jwelton at nautel.com
>
>
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