An analog signal
is any continuous
signal for which the time varying feature (variable
) of the signal is a representation of some other time varying quantity, i.e., analogous
to another time varying signal. For example, in an analog audio signal, the instantaneous voltage of the signal varies continuously with the pressure of the sound
waves. It differs from a digital signal, in which the continuous quantity is a representation of a sequence of discrete values which can only take on one of a finite number of values. The term analog signal usually refers to electrical signals; however, mechanical, pneumatic, hydraulic, human speech, and other systems may also convey
or be considered analog signals.
An analog signal uses some property of the medium to convey the signal's information. For example, an aneroid barometer
uses rotary position as the signal to convey pressure information. In an electrical signal, the voltage, current, or frequency
of the signal may be varied to represent the information.
Any information may be conveyed by an analog signal; often such a signal is a measured response to changes in physical phenomena, such as sound, light, temperature, position, or pressure. The physical variable is converted to an analog signal by a transducer
. For example, in sound recording
, fluctuations in air pressure (that is to say, sound) strike
the diaphragm of a microphone which induces corresponding fluctuations in the current produced by a coil in an electromagnetic microphone, or the voltage produced by a condenser microphone. The voltage or the current is said to be an "analog" of the sound.
An analog signal has a theoretically infinite resolution. In practice an analog signal is subject to electronic noise and distortion introduced by communication channels and signal processing operations, which can progressively degrade the signal-to-noise ratio
(SNR). In contrast, digital signals have a finite resolution. Converting an analog signal to digital form introduces a constant
low-level noise called quantization noise into the signal which determines the noise floor, but once in digital form the signal can in general be processed or transmitted without introducing additional noise or distortion. In analog systems, it is difficult to detect when such degradation
occurs. However, in digital systems, degradation can not only be detected but corrected as well.