The field of study that analyzes and deals with the operation of signals is called signal processing. The objects of study are the useful operations that can be done with signals. They can be used for control, identification, prediction, transmission and many other types of processes. Its theories fall within the discipline of applied mathematics but since it has so many practical applications, signal processing is engaged in various areas from computer science to telecommunications. Thus the principles of signal processing can be found underlying the mechanics of every type of electronic device man has engineered.
A signal is any measurable form of energy that can transmit information. This energy for example can be electrical, as in an electric current with a measurable property such as voltage. The measurable property of the energy varies across a certain period of time and these variations which may or may not be deliberate or controlled, can produce a certain pattern and thus convey information. Signals therefore do not exist in a vacuum; they are always part of signal systems. The field of study broadly defines signal systems as any physical configuration of components that can receive a signal which denotes input, and produce a signal which denotes output. Within a system, the latter is always a product or function of the former.
Time is the most common and significant factor for signals and this leads to two general classifications of signals - analog and digital. A digital signal is always a discrete-time signal and an analog signal is often a continuous-time signal. To illustrate the difference let us use an audio signal as an example. This signal is made up of continuously fluctuating voltages (thus analog/continuous-time). An electronic circuit can then be constructed to read the varying amount of voltage within constant and regular time intervals. What results is a series of quantities or data corresponding to discrete time values. This resulting data or output signal can now be called a digital signal.
Signals when they are inputted into a system are transformed or manipulated in order to perform a useful operation. Devices or circuits that carry out this transformation can generally be called signal processors. In the example given above, the electronic circuit is called an analog-to-digital converter. Another example of a signal processor is the circuitry inside a microphone. This circuitry can take in sound and convert it to fluctuating electrical voltages. These fluctuating voltages are said to be an analog of the received sound and in this form can be further manipulated or processed. They can be transmitted along a cable, strengthened through an amplifier and then outputted through a speaker. We call the whole physical configuration of microphone, cable, amplifier and speaker as an audio system.