—end of story. The estimated throughput of any all-digital AM schema is just 20 to 40 kilobits per second. This is not to say that I don't listen or that it's noise and affect isn't without it's charm. I listen to acetate recordings which we all know sound awful. The point is that anyone promising you "High Definition" or "High Fidelity" Digital AM radio is selling snake oil.
This service has been promised to us before. In fact in 2010 FCC approved IBOC (In-Band On-Channel) digital radio operation for both day and night AM broadcasts. You can see a list of the 300+ stations using this service that no one cares about here. You will notice that over half of the list is are distributed across just three media owners: Disney, Cumulus and Clear Channel. [Damn you Little John.] The important note here is that they have permission from the FCC. None of them actually broadcast in HD so far as I know.