Analog computers use continuous data representation and work directly with variables, while digital computers convert variables into numbers. Mechanical, hydraulic, electrical, and optical computers have been used for analog computing. Quantum computing also utilizes analog processing.
Any medium that can effectively represent a continuous state of data can serve as the basis for an analog computer. The earliest examples of analog computers were mechanical computers, which used moving parts and gears. Hydraulic computers and electric computers were developed later. In recent years, the principles of analog computing have been modeled using optical computers and have formed the basis for some emerging implementations of quantum computing.
An analog computer uses a means of representing the states of discrete variables. Unlike digital computers, which convert variables into a series of numbers, analog computers work directly with variables. They transform and conduct operations on variables using mechanical, electrical, or other processes to directly alter the state of the media representing the variable.
Mechanical analog computers have a long history. The Antikythera Mechanism, a product of Ptolemaic Egypt, modeled the movements of visible objects in the sky, using a complicated series of gears. Orreries served a similar function in the early modern period. Much more advanced mechanical analog computers were used during World War I and World War II to encode and decode data and to aid in targeting large artillery pieces.
Hydraulic computers model data using the flow of liquid rather than the movement of mechanical parts. Perhaps the earliest examples are the water clocks used during the Middle Ages. These used the constant flow and pressure of the water to accurately measure the passage of time. Most modern hydraulic analog computers use the flow of various liquids through closed systems to model the data.
Electrical analog computers use one of several properties of electricity to represent and manipulate data. The most common strains have historically used tension for this purpose. This type of analog computer is quite powerful and could outperform early digital computers when performing certain functions, but it was less versatile, as each new program typically required physical modification of the wiring and circuitry.
Analog processing can also be performed by optical computers. These computers encode information in the various properties of light and process the data by acting directly on this information. This type of processing can perform certain tasks, especially image analysis, with great efficiency.
The multistate nature of analog computing is also employed in the development of the first generation of quantum computers. Quantum computers can be structured to take advantage of the natural similarity between some types of physical structures and the underlying mathematical structure of some problems faced by quantum mechanics. As quantum computing emerges from its infancy, this type of computer tends to become more common and important.
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