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. 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.
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.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.
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