Taken from a press release announcing the approval of HD radio in Canada.:
"Using the HD Radio technology, existing AM and FM stations can broadcast digital signals along with the analog, providing multiple channels of programming on the same frequency, known as “multicastingâ€.
In addition to allowing for programming expansion via these multicasts, the technology’s developer, iBiquity Digital Corp., says that HD Radio provides improved sound quality over analog service. It can also allow for other neat options, like scrolling text and graphics content on the display of a receiver