SELCAL or Selective Calling is a function that allows a ground radio operator to alert an aircraft crew that the operator wishes to communicate with that aircraft. Because of the background noise level experienced on HF (High Frequency) radio frequencies, pilots usually prefer to turn down the audio level of their HF receiver until alerted via Selcal. When the ground station operator wants to communicate with the aircraft, they enter into the Selcal encoder the four letter code of that aircraft. This four letter code is usually included in the aircraft's flight plan and it is transmitted over the assigned radio channel. All aircraft monitoring that specific channel receive the Selcal broadcast but only those that have been programmed with that four letter code will respond by sounding a chime or some other alert for the crew.