Musical intervals are used as codes to communicate preselected commands or inputs to a computer. The system can use human voice sounding notes (DO-RE, etc.) successively to establish the interval and can use electrical circuitry and programs for converting the received notes or tones into pulse trains of the same fundamental period and for storing and calculating the period and interval. When a particular interval is received the apparatus including a digital micro-computer such as the Apple II PLUS.TM. may execute a pre-set recorded subroutine and restore the circuitry for the reception of a second interval.