Oscillators

In order to understand oscillators, you need to know a bit about sound. Sound consists of small, rapid fluctuations in air pressure. The air pressure increases slightly, then decreases slightly, then increases again, and so on. These fluctuations are called sound waves. When they strike our eardrums, we hear them.

The term “oscillator” comes from a Latin root that means “swing.” When a pendulum swings back and forth, it’s oscillating. So we can refer to the increases and decreases in air pressure as oscillations. If you pluck a guitar string that’s loosened so that it produces an extremely low pitch, you can actually see the back-and-forth motion (that is, the oscillations) of the string.

An oscillator, then, is an electronic device that produces a signal that is eventually converted into sound waves. The knobs and switches on an oscillator give us various kinds of control over the sound waves.

WAVEFORMS

All oscillators except the most basic ones have a menu, a rotary switch (such as the one in Apple’s ES2 soft synth, shown above), or some buttons with which you can choose one or another shape for the sound waves that will be produced. Sometimes you’ll see the word “waveshape,” but the more common term is “waveform,” which means the same thing.

The choice of one waveform or another is important because our ears are very, very good at detecting differences in the shape of sound waves. When the waveform changes even slightly, we hear the difference as a change in tone color.

In the early days of analog synthesis, most oscillators could produce only a few basic waveforms, which are illustrated in Figure 1. Today’s oscillators have a far wider palette of waveforms to choose from, but the basic types are still found. They’re especially useful for creating “vintage” sounds.

Some oscillators can play sampled (digitally recorded) waveforms. These waves can be quite complex and realistic-sounding.

TUNING

Most often, each oscillator in a synth has its own tuning controls. The coarse tuning knob changes the pitch of the oscillator in half-steps, and the fine tuning knob allows you to adjust the pitch up or down by smaller amounts.

Using the coarse tuning knob, you can tune two oscillators to a musical interval, or three oscillators to an entire one-finger chord. When two oscillators whose coarse tuning is the same are detuned from one another using the fine tuning parameter, you’ll hear a richer, more pleasing sound. The rich tone is the result of phase cancellation (also known as “beating”) among the overtones. Detuning is one of several techniques used to help the tone of oscillators sound less sterile.

WAVEFORM MODULATION

Instead of producing a tone that remains the same throughout the note or phrase, most oscillators allow the waveform to be modulated while you’re playing. Again, the goal is a less sterile, more expressive tone.

If the oscillator produces square waves, you’ll probably see a knob for adding pulse width modulation. The best way to understand this concept is probably to listen to it. Another common feature is FM (frequency modulation), in which the waveform from one oscillator changes the sound of another. A third way of changing the tone is waveform synchronization, better known as “sync.”

CATCH A WAVE

The best way to learn about waveforms is to shut off all of the effects in your synth and set the filters, envelopes, and other parameters to basic default values. Then step through the waveforms one at a time and listen to them.