Measuring Versus Counting
Two basic approaches to calculation have existed for centuries. An analog computer uses a physical quantity, such as a length, an angle or an electrical voltage, to stand for a number. The machine then lets nature do the arithmetic: if two lengths are laid end to end, their total is the sum. A digital computer instead holds numbers as separate symbols and manipulates them in discrete steps.
A simple everyday comparison is a clock. A traditional clock with hands moves smoothly, and you read the time from the position of the hands. A digital clock shows numbers that jump from one value to the next. The first is analog and the second is digital, although both tell the time.
Familiar Analog Machines
The slide rule is probably the best-known example. Its sliding scales are marked so that moving one against the other effectively multiplies numbers, and for much of the 20th century engineers and students relied on it. Its results are read off a scale, so the precision depends on how finely the marks can be read.
Older examples include the astrolabe, which modeled the sky for navigation and timekeeping, and the Antikythera mechanism, an ancient Greek geared device recovered from a shipwreck, which is generally understood to have modeled astronomical cycles. Mechanical tide-predicting machines of the 19th and early 20th centuries combined rotating parts to imitate the rise and fall of the sea.
The Differential Analyzer
Larger analog machines were built to solve differential equations, which describe how quantities change over time. A well-known example is the differential analyzer developed by Vannevar Bush and colleagues at the Massachusetts Institute of Technology in the late 1920s and early 1930s. It used rotating shafts and mechanical integrators, linked together to mirror the equation being studied, and several similar machines were built elsewhere.
Setting up such a machine for a new problem meant physically rearranging its parts, which could take a long time. This was a real limitation, but for certain problems the machine produced answers faster than a person could with pencil and paper.
Electronic Analog Computers
During the middle of the 20th century, electronic analog computers appeared. They used electrical voltages and amplifier circuits to imitate a system, for example the motion of an aircraft or the flow of a chemical process. The user connected modules with patch cords so that the circuit behaved like the system being modeled, then watched the result on a display or recorder.
These machines were used for computer simulation in engineering and in some military and research settings, particularly for continuous, real-time simulations. They were valued for their speed on suitable problems. Their accuracy, however, was limited by the quality of their components.
How Digital Machines Differ
A digital computer converts a problem into numbers and a list of steps. Early examples included mechanical calculators and the electromechanical machines and electronic computers of the 1930s and 1940s. These were usually built with binary numbers in mind, and they relied on logic gates to carry out arithmetic.
A key difference is how errors behave. In an analog device, small imperfections such as electrical noise, temperature changes or worn parts blend into the answer, and they accumulate as the calculation proceeds. A digital value, in contrast, is either one of its allowed states or it is not, so as long as the circuit can tell the states apart, small disturbances are ignored. This lets a digital machine repeat the same calculation and get the same result.
Trade-offs
Neither approach is better in every way.
- Precision: An analog reading is limited by how well a quantity can be measured. A digital machine can, in principle, carry as many digits as it has memory for.
- Flexibility: A digital computer changes its job by loading a new program. Many analog machines needed rewiring or reassembly.
- Speed on special tasks: A well-matched analog circuit can respond continuously and immediately, with no step-by-step processing.
- Repeatability: Digital results can be copied, stored and checked exactly.
Why Digital Took Over
Several developments favored digital design. The stored-program concept meant a single machine could take on many tasks. Transistors and, later, integrated circuits made digital hardware smaller and cheaper. Improvements in memory and storage allowed ever larger problems to be handled. As digital machines grew more capable, the cases where analog had a clear edge became fewer, and general-purpose digital computers became the standard.
The change was gradual, and there was a long period when both kinds were used side by side. Some systems combined them, using analog parts for continuous signals and digital parts for control, an arrangement often called a hybrid computer.
Analog Ideas Today
Analog technology did not vanish. Many everyday signals, such as sound waves and light, are continuous, so devices that capture them, like microphones and sensors, are analog at the point of contact. They convert the signal into numbers for a digital machine to process, and convert numbers back into a smooth signal at the end, as in a speaker.
Researchers have also revisited analog techniques for certain specialized tasks. Whether those approaches will matter widely is uncertain, and claims about them are best treated cautiously. The main historical point is that the question was never settled by one decisive moment. Digital design won out by being flexible, exact and increasingly inexpensive to build.