Russ Edmunds wrote: > *** And it is because IBOC is a hyrid, where the digital signals occupy > the sidebands of the channel and the analog the center that we > encounter the hissing. In turn, we have a hybrid here because, as Scott > indicated, broadcasters here obviously had some reservations about a > full digital conversion and also about using other bands ( although I > don't believe there was one single band available which could have > filled the bill. > > Bottom line, building one compromise onto another doesn't always result > in a good thing.
Well stated, as always, Russ. I do want to clarify one point from my earlier post, because it seems to be at least an occasional source of misunderstandings in these conversations. I believe Russ is referring to what I'll call digital self-interference, the sort of thing one might experience if listening to a strong IBOC signal (say, WHAM 1180, about 8 miles away from me with 50 kW ND-U) on a receiver with wide audio bandwidth (like, for instance, my Carver TX11b). On that radio - and on a VERY small number of other AM tuners with exceptionally good analog audio performance - WHAM's own digital sidebands cause enough interference to its own analog audio to make it unlistenable. I don't believe that sort of interference is common in real-world listening in 2007. I have something like 30 devices in my house that can tune AM radio, and of them, only the Carver, the three Superadios and the CC Radio display that sort of interference. On all but the Carver, it can be eliminated by switching to narrow mode. (The Carver still hears it in narrow mode, I think, because it's looking for an AM stereo signal that's transmitted with similar phase quadrature to the digital carriers - but now I'm getting way beyond my level of expertise on these things.) Sadly, as we all know, the "state of the art" for today's analog AM receivers is pretty pathetic - extremely narrow audio like the telephone-quality grunge I hear on the factory radio in my 2003 VW. One can wonder - and I do wonder - whether the hybrid system would ever have been considered acceptable if the average AM radio still in use today was a nice wideband model like the tube sets common 50 years ago. That sort of self-interference would certainly have been much more objectionable back then. But...I'm pretty sure that the much bigger interference concern from IBOC to analog, once nighttime operation begins, is adjacent-channel interference - WHAM's lower digital carriers stepping on WWVA's analog signal, for instance, or WCBS's upper carriers stepping on WLS analog. This sort of interference can't be eliminated just by going to a narrow audio bandwidth. The HD tuners I've used, especially the Sangean, take advantage of their DSP architecture to do a somewhat better job of filtering out that interference, at least on second-adjacent channels, but the laws of physics inevitably come into play here: there's only so much even the best DSP can do when an analog signal is swamped by high levels of incoming on-channel digital RF hash. Until and unless we get to an all-digital world on MW and SW - and I don't see that happening for a very long time - that sort of digital-into-analog interference is going to be a reality, and DRM isn't a complete solution. Imagine two scenarios: 1. I'm a Canadian station on 740 running analog-only. You're a high-power US station on 750 running IBOC. 2. I'm a small station somewhere in Europe on 1296 running analog-only. You're a high-power station somewhere else in Europe on 1296 running DRM. It seems to me the ultimate effect is the same: the digital hash is going to contribute at least some additional noise to reception of the analog signal that occupies the same spectrum. The difference between the two scenarios is ultimately purely regulatory: in Europe, presumably some effort has been made to coordinate interference between two stations that are licensed on the same channel, whereas in North America, the licensing scheme never envisioned the guy on 750 spreading that much RF energy down onto 740's slice of spectrum. That's ultimately an issue that will have to be resolved politically, not technologically. (Isn't that how so much of this story has gone already?) 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]
