Bluetooth
Bluetooth is a short-range wireless link used to send audio or calls between a phone, an adapter or a radio and a speaker.
Bluetooth is a short-range wireless connection. In radio use it carries audio from one device to another, for example from a phone to a speaker or from an adapter to a car radio, and it is used for hands-free calling.
Konsumentenschutz lists a smartphone over Bluetooth or USB as one way to keep listening in an older car. TCS noted that most of the adapters it tested offered Bluetooth hands-free calling.
Bluetooth does not receive radio by itself. It only moves sound that something else has received, whether that is a DAB+ adapter or an internet stream. Sound quality can differ from a cable connection.