Don Juannn said:
There is no factual basis to believe that anyone except DX-ers and hobbyists are doing so.
This thread centers on AM, so I'll talk about the AM situations we have in the San Francisco Bay Area. 1510, licensed to Piedmont, is quite strong in Walnut Creek and Concord. So is 1530 from Sacramento. Those stations are 20 KHz away from each other. But when 1530 turned on HD the 1510 signal picked up a sort of weird high-pitched noise. Most people had no idea what it was, but the engineers among us recognize it as a beat between the station's carrier and the ODFM carriers. The 2nd-adjacent interference is arguably more objectionable than 1st adjacent, because the desired station is still very clear. In our other big interference case, 910/920, the offending station (910) just drowns out 920 completely. This one is a bit worse than normal because they tried to shoehorn HD into a directional antenna system that wasn't really broadband enough to do it, so the HD signal doesn't get nulled the way it's supposed to.
FM is even worse in this hilly terrain. There are many instances of HD interference all over the Bay Area - even within the predicted city-grade contours. 107.5/107.7, 98.9/99.1, 92.1/92.3, and 91.5/91.7 come to mind. I agree that probably the DX-ers and hobbyists are the ones who know the problem is due to HD - you can add people who work in radio to that list. But the general public doesn't know any better, and they just switch to something else.
Dave B.