I should've mentioned this last week, but on Friday September 14th the FCC's new rules for IBOC digital radio (known in the US under the iBiquity brand name "HD Radio") went into effect. That meant, among other things, that IBOC was authorized for 24/7 operation on the AM band.
There's been a lot of controversy in the industry over nighttime IBOC. Anyone who's ever listened to AM during the day vs. during nighttime knows that distant (as in 100's or 1000's of miles distant) AM stations can come in like local stations at night. It's a quirk of the Earth's atmosphere called "skywave reflection". By and large this happens on the "clear channel" frequencies (no relation to the company of the same name) which only have one or two big stations in the entire continental US. These are stations like WBZ 1030 in Boston, WTIC 1080 in Hartford, and WOR 710 in New York...just to name a few.
AM IBOC intentionally puts a lot of RF energy on the adjacent channels of the transmitting frequency; that's just how the system works. So when you're in Boston and you tune an analog radio to 1020 or 1040, you'll hear a roar of white noise. That's the digital carriers...it's sort of like picking up the phone when a fax is transmitting. During the day it's not that big a deal because the AM signals stay local and locally there aren't going to be any stations on 1020 or 1040.
But at night...when these signals start reaching way beyond the local area due to skywave...there might be a station on 1020 or 1040. For example, KDKA in Pittsburgh...another "clear channel" station...is on 1020. So if these stations run IBOC and transmit it at night, will it result in interference?
A lot of engineering and even more speculation has been done on this issue. Ultimately the FCC decided that the answer was "For the most part, no" and authorized it. Radio purists cried foul rather loudly, but until it actually started happening, nobody really knew 100% for sure what was going to happen.
Which brings us to last Friday...when several of these bigger "clear channel" stations were just waiting to start nighttime IBOC broadcasting and immediately started doing so.
What's the result? Honestly I don't know. The invective is still spewing from both sides so I can't trust that, and I haven't been by a HD Radio-equipped tuner until today. I'll take a listen tonight and see what happens.
UPDATE 9/18: Meh, I'm not seeing massive AM disruption. Hell, I can't even get a solid HD signal here. I can get the HD light flashing on WTIC, WFAN and WOR, and even see a bit of PSD for WTIC, but that's it. Can't get a clear enough signal to hear IBOC audio, but that's no surprise...I couldn't do it before, either. I was a bit surprised that WHAM doesn't come in with IBOC here...the analog sounds fine. (shrugs) Anyways, the locals that I would expect to come in here, still come in just fine.
In all honesty, I kinda felt the AM band was a giant mess already...it's not really possible to make it worse. If anything, having more white noise/hash might be an improvement over the four overlapping skywave signals beating off each other.
Either way, my original point stands: we'll soon know definitively whether of not AM-IBOC at nighttime is a "disaster" or not...and the naysayers will either be vindicated or chastized. I fervently look forward to this particular debate being settled.