Re: [IRCA] recorded the iboc

[Scott Fybush <scott@xxxxxxxxxx>]

Patrick Martin wrote:

> What I was using this morning was the Drake SPR4 from 1971. A few months
> back when KGO was testing IBOC after Midnight, I heard the "Hash" under
> their signal on the R8 too. I did not try the R8 this morning however,
> but I would be surprised if it would have sounded any different. If the
> IBOC hash is being heard under KGO's audio on a Drake in 5-6 khz
> bandwidth, I would be surprised if everyone wouldn't hear it in the Bay
> area. KGOs signal is S9+30-50 DB typical and I can even hear the
> transmitter hum. So it is basically local like most nights. KGO even
> made the ratings #2 in the Arbitron Portland Book in the early 70s. That
> was before KPDQ was on, but KGO still has a decent signal at night in a
> lot of Portland. I just wonder what KGO's hash would do to KPDQ at 500w
> in the Portland area at night? 

You're mixing up two issues here, though.

The KPDQ issue is the IBOC sideband interfering with a first-adjacent 
analog signal. That's a very real concern, and of course it's the big 
obstacle to nighttime use of the system. For a station like KPDQ with a 
minimal (but protected class B) night signal, incoming interference from 
an IBOC sideband could do some significant harm.

The on-channel noise issue is also a real one, but it's fixable for most 
stations. Bill Harms is right that it's primarily a problem for stations 
with directional antennas. Your typical AM DA was designed anywhere from 
40 to 60 years ago with the idea that it would perform well over 10-12 
kHz or so of occupied bandwidth. Not all of those arrays will easily 
pass the significantly wider bandwidth of an AM IBOC signal without 
introducing unwanted phase distortion, particularly in the nulls, and 
it's that phase distortion that makes the hash noise audible on a 
typical AM receiver. With enough careful engineering work (and a fair 
amount of trial and error), most AM DAs - but not all - can be made 
broadband enough to pass a clean IBOC signal. WTWP has particularly 
tight nulls in its pattern, and it's not easy to tweak the array so it 
nulls the IBOC carriers above and below the analog signal without 
introducing distortion.

This is also the source of Pat's problem with KGO. Skywave, by its 
nature, distorts the phasing of incoming signals. That's why even the 
strongest skywave IBOC signals have yet to produce much recoverable 
audio for the DXers who've tried - and the same phase distortion that 
makes the audio unrecoverable also distorts the phase cancellation that 
should (when all's working well) make the digital signal inaudible on a 
normal analog radio. My educated guess is that the IBOC noise is thus 
much more objectionable when heard on strong skywave, as it would be at 
Seaside, than it would be on groundwave, as heard in KGO's home market.

Sound like an imperfect system? You bet it is. But those imperfections 
are still much more serious for DX listeners than for typical in-market 
listeners, and since we're dealing with a system that was designed with 
the assumption that there would be no DX listening, the flaws we're 
seeing here were, in a sense, designed into the system.

I'll reiterate that in my experience - and while I haven't been in 
either KGO's or WTWP's home markets since they turned on their IBOC, 
I've listened in a lot of other markets from Boston to LA to Seattle - I 
haven't heard audibly distracting background noise on any of the AM IBOC 
signals I've listened to, except on unusually wide-band AM receivers or 
AM stereo radios like the A100.

s
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