As for the use of digital audio on the analog AM band, that too has to be applied for. I'm aware of two stations operating with digital audio on AM. One in DC and the other in NY. I'm told their signals can be received on HD radios. They are not subscription based.
No, my idea isn't HD radio, it's similar in concept to the PC based Real Player.
In the late 1990s, I used the Real Player software w/dial-up Internet to listen to BBC 1 radio (in stereo), there was constant rebuffering so I lost interest in streaming radio listening until I got AT&T Fiber Internet last year.
My point is is that the Real Player software was able to provide reasonable quality stereo audio at dial-up data rates (using the noisy limited bandwidth analog landline) so a ~5kHz noisy analog AM radio signal should be able to contain enough bits to reproduce good quality stereo audio [using the latest low data rate audio codec + much, much error correction].
Kirk Bayne