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Pushback on HD Radio in AM: Citadel Turns off Nighttime AM
Criticisms of problems in using HD Radio encodings for AM stations date back to the earliest days of the technology: The FCC approved nighttime broadcasting of HD Radio over AM a few months ago, but there are still relatively few AM stations using HD Radio, and thus problems are only starting to emerge. At night, the so-called D region of the ionosphere changes character and reflects rather than dissipates signals in the AM radio range. AM radio signals already travel thousands of miles further than intended at night, and both hobbyists (DXers) who enjoy tuning in far-off stations, and regular radio listeners could be affected by digital signal skipping.
I've had a mixed opinion on this front, expecting that if there were real problems, then the FCC wouldn't approve nighttime broadcasts, that broadcasters wouldn't deploy such signals, and that lawsuits would be filed. I don't mean to be naive about the money involved, because broadcasters are equally concerned about interference that prevents their own stations from being heard.
Radio World reports that Citadel Broadcasting, which has HD Radio enabled on 16 of its 66 AM stations, decided to turn off the 10 stations that were broadcasting digital signals at night due to reports of interference. The problem occurs when listeners hear "hiss" and stations on nearby channels hear noise.
There have been rumors all along that AM digital encoding is inadequate and will need to be entirely rethought--that is, the underlying encoding replaced with a superior one. This would require replacement of all the HD Radio sets on the market, except those that have upgradability through various means (USB ports and discs that can upgrade firmware) almost certainly, as any solution would likely require new silicon.
We'll see what happens here. It's possible that in production HD Radio won't be able to work at night; that's always been a concern, and now they have to tweak parameters while still maintaining compatibility.
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