This can then be plugged into a portable audio player such as a Minidisc, CD or MP3 player. This is useful in older cars without CD changers, as many vehicles sold have a cassette player but no facility to play back these other formats. One can use their current device with the cassette deck, negating the need to purchase another digital audio player. Some cassette adapters use a tape head which simulates just one stereo pair of tracks of the cassette; to use these, the cassette player must be running in one specific direction. Other cassette adapters use a tape head which simulates both pairs of stereo tracks allowing the cassette player to run in either direction and still reproduce the input signals.
The device also contains some mechanics that fool auto-reversing cassette decks into thinking that there is an endless cassette tape in the deck. This mechanism usually consists of a drive belt that connects the take-up spindle to the supply spindle. Because this allows the supply spindle to always be turning, the cassette deck never senses "end of tape" and remains playing the same side of simulated cassette.