Glossary of Computer Science and Engineering Part 7 - Analog and Digital | HackTHatCORE

Analog and Digital

The word analog (derived from Greek words meaning “by ratio”) denotes a phenomenon that is continuously vari- able, such as a sound wave. The word digital, on the other hand, implies a discrete, exactly countable value that can be represented as a series of digits (numbers). Sound recording provides familiar examples of both approaches. Recording a phonograph record involves electromechanically transfer- ring a physical signal (the sound wave) into an “analogous” physical representation (the continuously varying peaks and dips in the record’s surface). Recording a CD, on the other hand, involves sampling (measuring) the sound level at thousands of discrete instances and storing the results in a physical representation of a numeric format that can in turn be used to drive the playback device. Virtually all modern computers depend on the manipu- lation of discrete signals in one of two states denoted by the numbers 1 and 0. Whether the 1 indicates the presence of an electrical charge, a voltage level, a magnetic state, a pulse of light, or some other phenomenon, at a given point there is either “something” (1) or “nothing” (0). This is the most natural way to represent a series of such states. Digital representation has several advantages over ana- log. Since computer circuits based on binary logic can be driven to perform calculations electronically at ever-increas- ing speeds, even problems where an analog computer better modeled nature can now be done more efficiently with digi- tal machines (see analog computer ). Data stored in digi- tized form is not subject to the gradual wear or distortion of the medium that plagues analog representations such as the phonograph record. Perhaps most important, because digi- tal representations are at base simply numbers, an infinite variety of digital representations can be stored in files and manipulated, regardless of whether they started as pictures, music, or text (see digital convergence ).

Converting between Analog and Digital Representations

Because digital devices (particularly computers) are the mechanism of choice for working with representations of text, graphics, and sound, a variety of devices are used to digitize analog inputs so the data can be stored and manip- ulated. Conceptually, each digitizing device can be thought of as having three parts: a component that scans the input and generates an analog signal, a circuit that converts the analog signal from the input to a digital format, and a com- ponent that stores the resulting digital data for later use. For example, in the ubiquitous flatbed scanner a moving head reads varying light levels on the paper and converts them to a varying level of current (see scanner ). This analog signal is in turn converted into a digital reading by an analog-to- digital converter, which creates numeric information that represents discrete spots (pixels) representing either levels of gray or of particular colors. This information is then written to disk using the formats supported by the operat- ing system and the software that will manipulate them.

References:

• Chalmers, David J. “Analog vs. Digital Computation.” Available online. URL: http://www.u.arizona.edu/~chalmers/notes/ana- log.html. Accessed April 10, 2007.
• Hoeschele, David F. Analog-to-Digital and Digital-to-Analog Conver- sion Techniques. 2nd ed. New York: Wiley-Interscience, 1994.