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Create sounds using analogue electronics (Part 2)

Posted: 08 Nov 2011     Print Version  Bookmark and Share

Keywords:sound synthesis  subtractive  acoustics 

Subtractive synthesis is usually mistaken as the only method of analogue sound synthesis. Although there are other methods of synthesis, the majority of commercial analogue synthesisers use subtractive synthesis. Because it is often presented with a user interface consisting of a large number of knobs and switches, it can be intimidating to the beginner.

Because there is often a one-to-one relationship between the available controls and the knobs and switches, it is well suited to educational purposes. It can also be used to illustrate a number of important principles and models that are used in acoustics and sound theory.

Theory: source and modifier
Subtractive synthesis is based around the idea that real instruments can be broken down into three major parts: a source of sound, a modifier (which processes the output of the source) and some controllers (which act as the interface between the performer and the instrument). This is most obviously apparent in many wind instruments, where the individual parts can be examined in isolation (figure 1).

For example, a clarinet, where a vibrating reed is coupled to a tube, can be taken apart and the two parts can be investigated independently. On its own, the reed produces a harsh, strident tone, whilst the body of the instrument is merely a tube that can be shown to have a series of acoustic resonances related to its length, the diameter of the longitudinal hole and other physical characteristic; in other words, it behaves like a series of resonant filters. Put together, the reed produces a sound which is then modified by the resonances of the body of the instrument to produce the final characteristic sound of the clarinet.

Although this model is a powerful metaphor for helping to understand how some musical instruments work, it is by no means a complete or unique answer. Attempting to apply the same concept to an instrument such as a guitar is more difficult, since the source of the sound appears to be the plucked string, and the body of the guitar must therefore be the modifier of the sound produced by the string.

Unfortunately, in a guitar, the source and the modifier are much more closely coupled, and it is much harder to split them into separate parts. For example, the string cannot be played in isolation in quite the same way as the reed of a clarinet can, and all of the resonances of the guitar body cannot be determined without the strings being present and under tension.

Figure 1: The performer uses the instrument controllers to alter the source and modifier parameters.

Despite this, the idea of modifying the output of a sound source is easy to grasp and it can be used to produce a wide range of synthetic and imitative timbres. In fact, the underlying idea of source and modifier is a common theme in most types of sound synthesis.

Subtractive synthesis
Subtractive synthesis uses a subset of this generalized idea of source and modifier, where the source produces a sound that contains all the required harmonic content for the final sound, whilst the modifier is used to filter out any unwanted harmonics and shape the sound's volume envelope. The filter thus 'subtracts' the harmonics that are not required; hence the name of the synthesis method (figure 2).

Figure 2: The source produces a constant raw waveform. The filter changes the harmonic structure, whilst the envelope shapes the sound.

Sources
The sound sources used in analogue subtractive synthesisers tend to be based on mathematics. There are two basic types: waveforms and random. The waveforms are typically named after simple waveshapes: sawtooth, square, pulse, sine and triangle are the most common. The shapes are the ones which are easy to describe mathematically and also to produce electronically. Random waveshapes produce noise, which contains a constantly changing mixture of all frequencies.

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