In signal processing, a Signal is a function that conveys information about a phenomenon. In electronics and telecommunications, it refers to any time varying voltage, current, or electromagnetic wave that carries information. It may also be defined as an observable change in a quality such as quantity.
Any quality, such as physical quantity that exhibits variation in space or time can be used as signals to share messages between observers. According to the IEEE, a signal can be audio, video, speech, image, sonar, and radar-related and so on. In another effort to define it, anything that is only a function of space, such as an image, is excluded from the category of signals. Also, it is stated that it may or may not contain any information.
Signals can be classified according to many criteria, mainly: according to the different feature of values, classified into Analog Signals and Digital Signals; according to the determinacy, classified into deterministic signals and random signals; according to the strength, classified into energy signals and power signals. Two main types of signals encountered in practice are analog and digital signals.
Signal Processing is the manipulation of signals. A common example is signal transmission between different locations. The embodiment of it in electrical form is made by a transducer that converts the signal from its original form to a waveform expressed as a current or a voltage, or electromagnetic radiation, for example, an optical signal or radio transmission. Once expressed as an electronic signal, it 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.