Signals

A signal is a function that conveys information about a phenomenon. In science and engineering, signals are quantities that vary in time or space and encode data such as electrical voltage, sound, light levels, or numerical samples. Signals are distinguished from noise, which obscures information. The study of signals and systems examines how signals can be measured, analyzed, and transformed to extract or modify information.

Signals can be continuous-time (analog) or discrete-time (digital). They can be deterministic or random (stochastic). Many practical signals are sampled for digital processing. The frequency content of a signal is described by its spectrum; transforms such as Fourier, Laplace, or z-transform are used for analysis and design. The sampling theorem relates sampling rate to reconstructability.

Signals occur across domains: in electronics and communications they carry information over channels; in controls they represent system states; in image and audio processing they form the data being processed. In biology, neural and physiological signals reflect body activity. In linguistics and social communication, signals can be verbal or nonverbal cues.

Signal processing aims to estimate, extract, filter, compress, or transmit signals. Common techniques include filtering to remove noise, modulation for transmission, coding to increase efficiency, and reconstruction after transmission. Performance is often judged by measures such as signal-to-noise ratio, distortion, and error rates.