Electric
Signals
A signal as referred to in communication systems, signal processing, and electrical engineering "is a function that conveys information about the behavior or
attributes of some phenomenon". In
the physical world, any quantity exhibiting variation in time or variation in
space (such as an image) is potentially a signal that might provide information
on the status of a physical system, or convey a message between observers, among other
possibilities. The IEEE Transactions on Signal
Processing elaborates upon
the term "signal" as follows: The term "signal" includes,
among others, audio, video, speech, image, communication, geophysical,
sonar, radar, medical and musical signals.
Other
examples of signals are the output of a thermocouple,
which conveys temperature information, and the output of a pH meter which conveys acidity information. Typically,
signals are often provided by a sensor,
and often the original form of a signal is converted to another form of energy
using a transducer. For example,
a microphone converts an acoustic signal to a voltage waveform, and a speaker does the reverse.
The formal study of the information content of
signals is the field of information
theory. The information in a signal is usually accompanied by noise. The term noise usually means an undesirable random
disturbance, but is often extended to include unwanted signals conflicting with
the desired signal (such as crosstalk).
The prevention of noise is covered in part under the heading of signal integrity. The separation of
desired signals from a background is the field of signal recovery, one branch of which is estimation theory, a probabilistic
approach to suppressing random disturbances.
Engineering
disciplines such as electrical
engineering have led the way in
the design, study, and implementation of systems involving transmission, storage, and manipulation of information. In the
latter half of the 20th century, electrical
engineering itself separated into
several disciplines, Specializing in the design and analysis of systems that
manipulate physical signals; electronic
engineering and computer
engineering as examples; while design engineering developed to deal with functional
design of man–machine interfaces.