Voltage control oscillation: The principle of voltage control was the most important development of early electronic music synthesizers. It is the characteristic that enables the output of a device to be determined by another device through the sending of a control voltage. The usual application is for the control of frequency or pitch or amplitude. An amplifier in a synthesizer provides control over the loudness of the sound but it also allows the amplitude to be determined by the output of an envelope generator Turning up the sustain level in the envelope generator and the amplifier will produce a sound which increases in volume at this part of the sound shape. A simple example where there are 2 oscillators in the synthesizer illustrates the principle well. The diagram below shows oscillator 2 (a square wave) set to a sub-audio pitch of 6 pulses a second. As the signal strength of oscillator 2 is increased, the vibrato audible in oscillator 1 will increase (the intervals high-low getting larger).