Classic Synthesizer: The Minimoog - voltage control
The key to synthesis, where a very wide range of sounds is possible, rather than the strictly limited range offered by electronic organs of the Minimoog's period, is in voltage control.
This means that every parameter, or at least most parameters, can be controlled from two sources, the knob on the front panel, and a voltage input. For instance, the Voltage Controlled Oscillator, or VCO, can be tuned from the front panel knob, but this can only produce unmusical swoops.
But get a keyboard to produce the voltage and the oscillator can produce a series of musical notes. Taking this a stage further and adding a low frequency oscillator, or LFO, can provide a modulating voltage which will create a vibrato effect - mild, rich or extreme in intensity.
Moving along the typical signal path of an analogue synthesizer we find a Voltage Controlled Filter, or VCF, which works in a very similar way: it can be set to whatever cut off frequency you wish on the front panel, and additionally it can take a control voltage input from the keyboard, so the cut off frequency canalways stay in the same proportion to the frequency of the note being played.
The third source of voltage control for either the VCO or VCF is the Envelope Generator which produces a voltage signal which ramps up and down to create the Attack-Decay-Sustain-Release envelope with which we are familiar from modern digital synthesizers although modern synths have more complex envelopes which may improve the overall sound but do little for controllability.
The Envelope Generator may control the pitch of the note being played, the cut off frequency of the filter, or alternatively the Voltage Controlled Amplifier so that the note played has a defined attack, decay etc.