A synthesizer is a collection of simple electrical modules that create sound when properly used together. Analog Synthesizers started out as big modular beasts because the different modules needed to make sound were invented one at a time as add-ons to already existing gear, and because the technology at the time wasn't as intergrated as today. Their immense size gave these early synths the impression of being very complex when in fact they were not. Today's analog synths are much more compact single box solutions but all the basic principles are the same. Here's a simple synth cicuit.

The first module in the synth chain is the oscillator. This is a simple circuit that produces a tone when it is turned on. It has two separate kinds of power coming to it. The first powers the circuit itself and the second is used to make the circuit change its pitch. You vary the second voltage in small increments form 0 to 15 volts to go from the lowest to highest pitch. This is where the term VCO comes from. It stands for voltage controlled oscillator.

So an oscillator is the basic tone generator in your synth. It makes the original noise and you can change it's pitch by noodling with this other voltage that controls it. You could make a small keyboard with a series of little resistors at each key set up so that the lowest key provides 15 volts of resistance while the highest lets the current through. Then you could run the voltage controlling pitch through it first then to the oscillator. If you did this hitting different keys would make the oscillator change it's pitch accordingly.

This is basically how a keyboard in an old analog synth is wired. The problem is however that the oscillator is always on and always makes a sound. What you need is a way to turn the sound off when you let go of a key without turning the actual oscillator off. The circuit that achieves this is called a voltage controlled amplifier (VCA).

If you understood VCO you know what VCA actually means. The sound emitted by the VCO goes through the VCA. The VCA is a small amplifier circuit. It has two types of control. One is just like the VCO control but instead of pitch it controls the overall volume of the sound going out. The other is a simple switch that either lets the sound through or not. This second part of the VCA is called a gate.

I f you were to wire the gate leads from the VCA to the little keyboard we were discussing earlier in such a way that when you press a key it makes contact then you would now have a finished basic synthesizer.

Now what happens is that no sound comes out of the synth unless you press a key. When you do press a key the VCO changes it's pitch according to what voltage it receives. The contact you make when pressing the key causes the gate to let the sound out. pretty neat huh ?

Of course there are other ways to make a synthetic noise but most techniques own their roots to this analog circuit.