My new car supports HD Radio. What is that? It’s a US standard for radio stations to broadcast a digital sideband adjacent to their audio frequency. The FM version is a 100-150 kbit/sec digital stream that encodes data in HDC, a proprietary standard that is similar to AAC. In theory there can be multiple HD streams on one channel, and a station could be digital-only. I haven’t seen that in practice.
The audio quality definitely seems better than analog FM. It’s also better than the low bandwidth Sirius XM channels like their NPR channel, that thing is awful. The big bummer is that since it’s a sideband the digital FM broadcasts don’t really offter the opportunity to free up any spectrum. Not that the TV spectrum process has gone very well.