Signal (electrical engineering)

In the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying quantity. 

The concept is broad, and hard to define precisely.  Definitions specific to subfields are common. For example, in information theory, a signal is a codified message, i.e., the sequence of states in a communications channel that encodes a message. In a communications system, a transmitter encodes a message into a signal, which is carried to a receiver by the communications channel.  For example, the words "Mary had a little lamb" might be the message spoken into a telephone. The telephone transmitter converts the sounds into an electrical voltage signal. The signal is transmitted to the receiving telephone by wires; and at the receiver it is reconverted into sounds.

 Examples of signals 
 Motion.  The motion of a particle through some space can be considered to be a signal, or can be represented by a signal.  The domain of a motion signal is one-dimensional (time), and the range is generally three-dimensional.  Position is thus a 3-vector signal; position and orientation is a 6-vector signal.

 Sound.  Since a sound is a vibration of a medium (such as air), a sound signal associates a pressure value to every value of time and three space coordinates.  A microphone converts sound pressure at some place to just a function of time, using a voltage signal as an analog of the sound signal.

 Compact discs (CDs).  CDs contain discrete bits representing a sound signal, recorded at 44,100 samples per second.  Each sample contains data for a left and right channel, which may be considered to be a 2-vector (since CDs are recorded in stereo).

 Noise typically unwanted but not always.

References
Shannon, C. E., 2005 [1948], "A Mathematical Theory of Communication,"  (corrected reprint ), accessed Dec. 15, 2005. Orig. 1948, Bell System Technical Journal, vol. 27, pp. 379–423, 623-656.

Category:Electronics
Category:Electrical engineering
Category:Telecommunications