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 _______________________________________________ IRCA mailing list [email protected] http://montreal.kotalampi.com/mailman/listinfo/irca Opinions expressed in messages on this mailing list are those of the original contributors and do not necessarily reflect the opinion of the IRCA, its editors, publishing staff, or officers For more information: http://www.ircaonline.org To Post a message: [email protected]
