Signals are physical variables that carry information about a particular process or event of interest. Signals are defined mathematically over a range and domain of interest and constitute different things to different people. To an electrical engineer, it may be.
• A current
• A voltage
• Power
• Energy To a mechanical engineer, it may be
• A force
• A torque
• A velocity
• A displacement to an economist, it may be.
• Growth (GNP)
• Employment rate
• Prime interest rate
• Inflation rate.
• The stock market variations to a meteorologist, it may be
• Atmospheric temperature.
• Atmospheric humidity
• Atmospheric pressures or depressions.
• Wind speed to a geophysicist, it may be
• Seismic waves
• Tsunamis
• Volcanic activity
To a physician, it may be.
• An electrocardiogram (EKG)
• An electroencephalogram (EEG)
• A sonogram for a telecommunication engineer, it may be
• Audio sound wave (human voice or music)
• Video (TV, HDTV, teleconference, etc.)
• Computer data.
• Modulated waves (amplitude modulation [AM], frequency modulation [FM], phase modulation [PM], quadrature amplitude modulation [QAM], etc.)
• Multiplexed waves (time division multiplexing [TDM], statistical time division multiplexing [STDM], frequency division multiplexing [FDM], etc.)
From a block box diagram point of view, signals constitute inputs to a system, and their responses referred to as outputs. Since many of the measuring, recording, tracking, and processing instruments of signal activities are electrical or electronic devices, scientists and engineers usually convert any type of physical variations into an electrical signal.
Electrical signals can be classified ed using a variety of criteria. Some of the signal’s classify cation criteria are.
a. Signals may be functions of one or more than one independent variable generated by a single source or multiple sources.
b. Signals may be single or multidimensional.
c. Signals may be orthogonal or nonorthogonal, periodic or nonperiodic, even, odd, or present a particular symmetry.
d. Signals may be deterministic or nondeterministic (probabilistic).
e. Signals may be analog or discrete.
f. Signals may be narrow or wide band.
g. Signals may be power or energy signals.
In any case, signals are produced as a result of a process defi ned by a mathematical relation
usually in the form of an equation, an algorithm, a model, a table, a plot, or a given rule.
A one-dimensional (1-D) signal is given by a mathematical expression consisting of one
independent variable, for example, audio. A 2-D signal is a function of two independent
variables, for example, a black and white picture. A full motion black and white video can
be viewed as a 3-D signal, consisting of pictures (2-D) that are transmitted or processed at
a particular rate. The dimension of a video signal can be increased by adding color (red,
green, and blue), luminance, etc.
