Signal

Signal refers to both the process and the result of transmission of data over some media accomplished by embedding some variation.

[1] Any quantity that can vary over space or time can be used as a signal to share messages between observers.

[a] In nature, signals can be actions done by an organism to alert other organisms, ranging from the release of plant chemicals to warn nearby plants of a predator, to sounds or motions made by animals to alert other animals of food.

For example, a microphone converts an acoustic signal to a voltage waveform, and a speaker does the reverse.

Information theory serves as the formal study of signals and their content.

The reduction of noise is covered in part under the heading of signal integrity.

The separation of desired signals from background noise is the field of signal recovery,[5] one branch of which is estimation theory, a probabilistic approach to suppressing random disturbances.

Definitions specific to sub-fields are common: Signals can be categorized in various ways.

Particularly in digital signal processing, a digital signal may be defined as a sequence of discrete values, typically associated with an underlying continuous-valued physical process.

Two main types of signals encountered in practice are analog and digital.

For example, an aneroid barometer uses rotary position as the signal to convey pressure information.

Alternatively, a digital signal may be considered to be the sequence of codes represented by such a physical quantity.

[13] The physical quantity may be a variable electric current or voltage, the intensity, phase or polarization of an optical or other electromagnetic field, acoustic pressure, the magnetization of a magnetic storage media, etc.

With digital signals, system noise, provided it is not too great, will not affect system operation whereas noise always degrades the operation of analog signals to some degree.

Digital signals often arise via sampling of analog signals, for example, a continually fluctuating voltage on a line that can be digitized by an analog-to-digital converter circuit, wherein the circuit will read the voltage level on the line, say, every 50 microseconds and represent each reading with a fixed number of bits.

The resulting stream of numbers is stored as digital data on a discrete-time and quantized-amplitude signal.

Deterministic signals are those whose values at any time are predictable and can be calculated by a mathematical equation.

In the mathematical abstraction, the domain of a continuous-time signal is the set of real numbers (or some interval thereof), whereas the domain of a discrete-time (DT) signal is the set of integers (or other subsets of real numbers).

As a result, the values of such a signal must be quantized into a finite set for practical representation.

Once expressed as an electronic signal, the signal is available for further processing by electrical devices such as electronic amplifiers and filters, and can be transmitted to a remote location by a transmitter and received using radio receivers.

Depending on the school, undergraduate EE students generally take the class as juniors or seniors, normally depending on the number and level of previous linear algebra and differential equation classes they have taken.

[19] The field studies input and output signals, and the mathematical representations between them known as systems, in four domains: time, frequency, s and z.

It involves circuit analysis and design via mathematical modeling and some numerical methods, and was updated several decades ago with dynamical systems tools including differential equations, and recently, Lagrangians.

Students are expected to understand the modeling tools as well as the mathematics, physics, circuit analysis, and transformations between the 8 domains.

have very close analogies in signal science (inductance, resistance, voltage, etc.

have now been applied to signals, circuits, systems and their components, analysis and design in EE.

Dynamical systems that involve noise, filtering and other random or chaotic attractors and repellers have now placed stochastic sciences and statistics between the more deterministic discrete and continuous functions in the field.

(Deterministic as used here means signals that are completely determined as functions of time).

EE taxonomists are still not decided where signals and systems falls within the whole field of signal processing vs. circuit analysis and mathematical modeling, but the common link of the topics that are covered in the course of study has brightened boundaries with dozens of books, journals, etc.

called "Signals and Systems", and used as text and test prep for the EE, as well as, recently, computer engineering exams.