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Quick Guide To Oscillators

Oscillators form the core of many synthesis systems. We get good vibrations as we explain how they work and what they do...

Virtually every form of synthesis (with the possible exception of physical modelling popularised in Yamaha's range of VL synths) follows three main stages: tone generation > tone shaping > volume shaping. The tone generation section can take several forms but the most common is the oscillator which is the main sound source in analogue synthesisers and software emulations.

Oscillators generate sound by, er, oscillating. That is, their circuitry basically changes or oscillates between two states very quickly, and just as a vibrating string produces a sound, so the oscillating electronic circuit generates a waveform that can be amplified and used as a sound source. And that's as technical as we're going to get here!

The output of an oscillator has three parameters - frequency or pitch, amplitude or volume and waveform or tone. We can see how they relate to a sound by looking at the simplest of all waveforms, a sine wave.

Give me a sine

Oscillators produce repetitive or cyclic waveforms which are usually measured in Hertz (abbreviated to Hz). The faster the oscillator vibrates, the more cycles there will be in a given time and the higher the frequency or pitch will be.

Here we have one full cycle of a sine wave. The distance from the highest to lowest points (technically, the peaks) is its amplitude and the greater the amplitude, the louder it will be.

You can see the differences between waveforms of different amplitudes and frequencies in this illustration. The top two waveforms have exactly the same number of cycles but the peak-to-peak range - the amplitude - of the top one is greater so it will be louder. The bottom waveform has the same amplitude as the top one so they will have the same volume, but it has more cycles so its pitch will be higher.

In control

One of the main features of analogue synthesisers is the way the various modules pass messages to each other. It's all done with voltages. When you press a key on an analogue synth it sends a specific voltage to the oscillator which generates a particular pitch. Higher and lower keys produce higher and lower voltages and correspondingly higher and lower notes. Analogue oscillators, therefore, are commonly known as VCOs or Voltage Controlled Oscillators.

Voltage control was used with synth modules such as oscillators, filters, amplifiers and envelope generators. It became such a natural and relatively intuitive method of control that even now, many digital synthesisers emulate the VCO routing concept sometimes quite specifically as in Reason's Control Voltage cabling. Even though digital oscillators don't generate waveforms in the same way as analogue synths, many soft synths adopt analogue synth methods and terminology.

Nice tone

The tone of a waveform is determined by the harmonics it contains. Harmonics are additional frequencies higher than the fundamental and usually at a lower volume  Harmonics have a natural mathematical relationship to each other - the 2nd harmonic is twice the frequency of the fundamental, the 3rd harmonic is three times the fundamental frequency, the 6th harmonic is six times the frequency and so on. The science of sound doesn't get much easier than this!

The harmonics in a waveform determines its shape. We can easily see the amplitude and frequency of a wave by looking at it. However, each waveform has its own harmonic content and its own tone, and it's virtually impossible to know what it will sound like by looking at it. The best approximation we can do is to say that the more complex a waveform is, the more likely it is to have lots of harmonics and sound richer, certainly in comparison to a sine wave.