> But I'm curious on the critical hours thing, too. DE.
I thought that would have been obvious.
As we know, HD on AM is limited to daylight hours. Further, as the sun begins to set before offical sunset, skywave is possible on MW (hence, "critical hours").
Now, we all know how HD works, spilling onto other channels. It may well be that as the skywave picks up, the HD signals will land somewhere where it is causing CCI. This is particularly concerning where that signal is causing interference on a "Clear" given to another country (e.g., Mexico, Canada, Bahamas). In this case, matters of international treaty would arise, which would trump FCC Rulemaking (Supremecy Clause, and all that). I am sure domestic broadcasters might also be a bit miffed.
If the main carrier were inaudible for whatever reason (most likely, interference on the main channel), it might well be appropriate to ID the HD channel.
But, I ain't a Commissioner.
I am not an opponent of IBOC, but I recognize that interesting issues can arise.
As an aside, though, I have not heard anyone comment on how well HD AM survives a hop off the ionosphere. As those schooled on such esoteric issues know, refraction through the ionosphere can do all sorts of odd things to a signal, and digital radio might be prone to dropping off the cliff in such cases. A few FM hobbyists this Summer, though, have heard usable signals from HD FM signals while they were bouncing off sporadic-E. But, is this analogous?
DE