Digital Radio

Digital Radio is different around the world, In most of Europe they use DAB+. MP4 the whole signal turns of and on in pulses to encode the characters in the data stream. This allows for more stations to be crammed into smaller bandwidths. The problem with this is you need to buy new radios to pick up the signal and the micro chip in the radio is working hard to decompress the signal. 

In the United States they use FM HD. In FM HD, the digital signal is hidden on either side of the FM frequency. So every FM station can have additional digital stations. It works a bit like Teletext. The signal can't be perceived in the FM but it is there and contains the digital sound codec. This system allows more FM stations to persist and still provide digital stations.  Ordinary FM transistor radios will still pick up the FM signal but you need new radios to pickup the additional digital stations.