Analog computer

An analog computer is a form of computer using electronic or mechanical phenomena to model the problem being solved by using one kind of physical quantity to represent another.

The term is used in distinction to digital computers, in which physical or mechanical phenomena are used to construct a finite-state machine which is then used to model the problem being solved. There is an intermediate group, Hybrid computers, in which a digital computer is used to control and organize inputs and outputs to and from attached analogue devices; for instance analogue devices might be used to help generate initial values for iterations.

Some examples:

• the abacus is a hand-operated digital computer
• the slide rule is a hand-operated analog computer

Computations are often performed, in analog computers, by using properties of electrical resistance, voltages and so on. For example, a simple two variable adder can be created by two potentiometers in series. The first value is set by the first pot (say x ohms), and the second value is set to the second pot (say y ohms). Measuring the resistance across the two pots will give the sum in resistance x+y ohms. Other calculations are performed similarly, using operational amplifiers and other circuits for other tasks.

Electronic analog computers are generally limited by noise and bandwidth considerations, and have been replaced by digital computers for almost all calculations. It may be stretching a point to regard physical simulations such as wind tunnels as analog computers.

A simple form of analog computation still in regular use is the nomogram.

Practical analog computers

These are examples of analog computers that have been constructed or practically used:

• astrolabe
• operational amplifier
• disk planimiter
• mechanical integrator
• Tide predictors
• Water integrator
• Target Data Computer
• Hydraulic model of UK economy
• the Antikythera mechanism
• and (trivially) the slide rule

Analog synthesizers can also be viewed as a form of analog computer, and their technology was originally based on electronic analog computer technology.

Idealised analog computers

Computer theorists often refer to idealised analog computers as real computers (so called because they operate on the set of real numbers). These idealised computers can in theory enable solve problems that are inextricable on digital computers.

Real analog computers are far from attaining this ideal, with noise and other errors completely swamping any hypothetical computation-theoretic advantages.

Also see signals, set theory, computability theory, differential equation, dynamical systems, chaos theory.