The MIDI VCO (NCO model) is an oscillator that produces a square wave whose
frequency is based on a digital input. The input must consist of seven bits
(MIDI note). MIDI notes are numbered between zero and 127 (Bit 1 is the MSB).
Note: number zero corresponds to a C 5 octaves below middle C. There are 12
notes per octave, so a middle C (which is 261.62 Hz) is note number 60. A440
(A above middle C) is note number 69, and so forth. Square waves of different
frequencies can be produced by changing the bit pattern and the mult_factor.