The best way I can describe HD radio is this. What you hear on an regular FM radio only takes up a small portion of the frequency. Many years ago the unused portion of the freq. was used for pagers, but pagers are so "70s", and nobody uses them anymore. So, that leaves a new, unused portion. What you hear on a regular radio is analog. Digital signals take up considerably less space, but provide much more information. On FM, up to 3 different programs can be selected on any given frequency. On AM, there is just enough room for 1. And the best part is that it's free. I have a Sony car radio that retails for about $170.00, and the HD module was $100.00
So, Oddball, in the Metro Atlanta area, there are 24 FM, and 3 AM stations broadcasting HD radio. Not all stations broadcast multiple programming, but the difference of digital vs analog is noticeable, drastically noticeable on AM. Literally CD quality.
HDradio.com. Check it out. I'm about to buy a new one and put the old one in my spare car.