[R-390] IBOC - was R-390a Image problem?

[Tim Shoppa]

Roy wrote:
>>What is "IBOC"?

> I am as yet ignorant about IBOC.  I see that it means In Band On Channel 
> (Digital) transmission.  I suspect this will be trouble.

It's not all that bad.

On any AM broadcast signal, the FCC limits the bandwidth to +/-
10kc. So a station at 680kc will nominally own the spectrum from 
670kc to 690kc. The FCC allocates frequencies every 10kc, so
around a big-power 680kc station the 670 and 690 kc slots were
not allocated.

On a non-IBOC station, very little power from the 680kc station
will be below 675kc or above 685kc. Yes, that bandwidth is available,
but most program content is and most transmitted power is in the few kHz range.
So even though you might have a 680kc station locally, not much power is
at 670kc, and you might be able to pretty regularly receive some
BCB DX on 670kc especially if you tune to the lower sideband and
have good filters in your receiver.

But on an IBOC station, they cleverly send digital data (which sounds
like "hash" if you listen to it) in this not-very-much-used bandwidth.
With an AM detector centered on the IBOC station, you don't notice
the digital data much because somehow they send it in the sidebands
such that it cancels out in the AM detector. But all of a sudden the
previously mostly-empty sidebands are now filled with IBOC signals.
Of course, to you and me the IBOC "signal" sounds like horrific noise!

But if you're trying to listen to that 670kc BCB DX station, the
clever cancellation doesn't exist anymore. You do hear the noise.

>  I un -subscribed quickly.  The various opinions of those folks 
> seemed to be:
>  - IBOC is the thing of the future. Gotta have it. Keep up with Progress!
>  - IBOC it devilishly difficult to set up and adjust at the station
>  - Careful engineering and diligent attention to the details will solve 
all those problems.
>  - You (the station) need a whole new transmitting antenna to make it work 
(AM stations especially?)

Engineering, especially the extra power being put into the sidebands,
does require technical upgrades to a lot of existing AM broadcast
stations. I don't know if this requires antenna changes (although I
do remember some tidbits about TV antennas being unable to
handle the full power bandwidth!) but it
could. Most of what I hear is about transformers in the transmitter.

There is some hope that IBOC's technical requirement will
put some extra effort into actual engineering at the blowtorch
stations that are adapting it. But even this has a downside:
several big ham power suppliers are rumored to be cutting
sales to hams as they devote more effort towards the
equipment the broadcast stations are upgrading to. This is
one reason I've heard hams give for why, for example, Peter
W Dahl transformers are so hard to buy lately.

Tim.