4 Computer Cousins — Analog and Digital
There are many thousands of computers in operation today—in enough different outward varieties to present a hopeless classification task to the confused onlooker. Actually there are only two basic types of computing machines, the analog and the digital. There is also a third computer, an analog-digital hybrid that makes use of the better features of each to do certain jobs more effectively.
The distinction between basic types is clear-cut and may be explained in very simple terms. Again we go to the dictionary for a starting point. Webster says: “Analogue.—That which is analogous to some other thing.” Even without the terminal ue, the analog computer is based on the principle of analogy. It is actually a model of the problem we wish to solve. A tape measure is an analog device; so is a slide rule or the speedometer in your car. These of course are very simple analogs, but the principle of the more complex ones is the same. The analog computer, then, simulates a physical problem and deals in quantities which it can measure.
Some writers feel that the analog machine is not a computer at all in the strict sense of the word, but actually a laboratory model of a physical system which may be studied and measured to learn certain implicit facts.
The dictionary also gives us a good clue to the digital computer: “Digital.—Of the fingers or digits.” A digital machine deals in digits, or discrete units, in its calculations. For instance, if we ask it to multiply 2 times 2, it answers that the product is exactly 4. A slide rule, which we have said is an analog device, might yield an answer of 3.98 or 4.02, depending on the quality of its workmanship and our eyesight.
The term “discrete” describes the units used by the digital machine; an analog machine deals with “continuous” quantities. When you watch the pointer on your speedometer you see that it moves continuously from zero to as fast as you can or dare drive. The gas gauge is a graphic presentation of the amount of fuel in the tank, just as the speedometer is a picture of your car’s speed. For convenience we interpolate the numbers 10, 20, 30, 1/4, 1/2, and so on. What we do, then, is to convert from a continuous analog presentation to a digital answer with our eyes and brain. This analog-to-digital conversion is not without complications leading to speeding tickets and the inconvenience of running out of gas far from a source of supply.
A little thought will reveal that even prior to computers there were two distinct types of calculating; those of measuring (analog) and of counting (digital). Unless we are statisticians, we encounter 2-1/2 men or 3-1/2 women about as frequently as we are positive that there is exactly 10 gallons of fuel in the gas tank. In fact, we generally use the singular verb with such a figure since the 10 gallons is actually an arbitrary measurement we have superimposed on a quantity of liquid. Counting and measuring, then, are different things.
Because of the basic differences in the analog and digital computers, each has its relative advantages and disadvantages with respect to certain kinds of problems. Let us consider each in more detail and learn which is better suited to particular tasks. Using alphabetical protocol, we take the analog first.