Analog computers tend to be used for special purpose applications which involve controlling manufacturing or similar "real-time" processes. They measure values using a continuous range (typically, a continuous range of electrical voltages) to manipulate environmental characteristics such as temperature and gas pressure. There has been a tendency over the past 10 or 20 years to replace this type of computer with digital computers which are connected to the "real-world" with "analog-to-digital" interface devices.
Digital computers are, by far, the more common type. They are the only type which we will consider in this course.
The main idea behind a digital computer is that, at least at a low level, values are represented using "descrete", rather than "continuous" units. Instead of a value being represented by any possible voltage between 0 volts and 5 volts (for example), a digital computer might require that the value be represented by one of the voltages: 0 volts, 1.5 volts, 3.0 volts, or 4.5 volts.
Some early digital computer systems used a 10-level representation scheme (which made working with our "normal" base 10 number system easy). Various other numbers of levels have been attempted with varying degrees of success. Modern digital computers, however, are restricted to a simple 2-level, or "binary" system.
As currently implemented, a "digital computer" is synonymous with a "computer which uses a binary system for value representation".
(examples of codes)
(reasons for using codes). ..communication ..internal representation ..encryption (anti-open communication)
concept of domain; a piece of information identifies a particular possible entity within a domain; example, domain:season, entities: spring, summer, fall, winter; 4 codes are required; codes are not required for entities which are not part of the domain (no code is needed for "tree" or for 47).
coding system: combination of encoding and decoding schemes; one-to-one
entities may belong to many coding systems (same entity, different codes). codes may belong to many coding systems (same code, different entity)
encoding systems with no corresponding decoding systems
codes composed of multiple binary values
a code identifies a particular entity within a domain. in this sense a code provides "information" and not just "data" however....
in practice, we will use the terms interchangeably.