3.2 Analogue and digital
The word 'analogue' means that a range of values are presented in a continuous rather than a discrete way. 'Continuous' implies making measurements all the time, and also infinite resolution – although inherent physical limitations such as the grain size on photographic film or the noise level in an electronic circuit will prevent any real-world system from being truly continuous. 'Discrete' means that you use individual finite sample values taken at regular intervals rather than measure all the time, with the assumption that the samples are a good representation of the original signal. Digital synthesis uses these discrete values.
The word 'analogue' can also be spelt without the '-ue' ending. In this book, the longer version will be used.
An analogue synthesizer is thus usually defined as one that uses voltages and currents to directly represent both audio signals and any control signals that are used to manipulate those audio signals. In fact, 'analogue' can also refer to any technology in which sound is created and manipulated in any way where the representation is continuous rather than discrete. Analogue computers were used before low-cost digital circuitry became widely available, and they used voltages and currents to represent numbers. They were used to solve complex problems in navigation, dynamics and mathematics.
Analogue electronics happens to be a convenient way of producing sound signals – but there are many other ways: mechanical, hydraulic, electrostatic, chemical, etc. For example, vinyl discs use analogue technology where the mechanical movement of the stylus is converted into sound. Tape recorders reproduce sound from analogue signals stored on magnetic tape.
In synthesizers, the use of the word 'analogue' often implies voltage-controlled oscillators (VCOs) and filters (VCFs). These have a set of audio characteristics: VCOs can have tuning stability or modulation linearity problems, for example; and analogue filters can break into self-oscillation or may distort the signal passing through them. These features of the analogue electronics that are used in the design can contribute to the overall 'tone quality' of the instruments.
Analogue synthesizers are commonly regarded as being very useful for producing bass, brass and the synthesizer 'cliché' sounds, but not a very good choice for simulating 'real' sounds. The typical clichéd sound is usually a 'synthy' sound consisting of slightly detuned oscillators beating against each other, with a resonant filter swept by a decaying envelope.
In contrast, digital synthesizers use discrete numerical representations of the audio and control signals. They are thus capable of reproducing prerecorded samples of real instruments with a very high fidelity. They also tend to be very precise and predictable, with none of the inherent uncertainty of analogue instruments. Some of the many digital synthesis techniques are described in Chapter 5.
Digital synthesizers can deliberately introduce randomness, of course!
The difference between analogue and digital representations can be likened to an experiment to measure the traffic flow through a road junction. The actual passage of cars can be observed and the number of cars passing a specific point in a given time interval are noted down. The movement of the cars is analogue in nature since it is continuous, whereas the numbers are digital since they only provide numbers at specific times (Figure 3.2.1).
FIGURE 3.2.1 The movement of the cars is continuous or analogue, whereas the number of cars is discrete or digital.
This link between a physical experiment and the numbers, which can be used to describe it, is also significant because the first analogue synthesizers, and in fact the first computers, were analogue not digital. An analogue computer is a device that is used to solve mathematical problems by providing an electrical circuit which behaves in the same way as a real system, and then observing that happens when some of the parameters are changed. A simple example is what happens when two containers filled with water are connected together. This can be modelled by using an integrator circuit: a capacitor in a feedback loop (Figure 3.2.2).
FIGURE 3.2.2 Two connected buckets can model an integrator circuit. 3.2 Analogue and digital
A step voltage applied to the integrator input simulates pouring water into one container – the voltage at the output of the integrator will rise steadily until the voltage is the same as the applied voltage, and then stops. If the integrator time constant is made larger, which is equivalent to reducing the fl ow of water between the containers (or making the second container larger), then the integrator will take longer to reach a steady state after a step voltage has been applied.
More sophisticated situations require more complex models, but the basic idea of using linear electronic circuits to simulate the behavior of real-world mechanical systems can be very successful. For more information on modelling techniques, see Section 5.3.