Re: [IRCA] DRM vs IBOC

[Scott Fybush <scott@xxxxxxxxxx>]

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
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