Signals can be categorized in various ways. The most common distinction is between discrete and continuous spaces that the functions are defined over, for example, discrete and continuous-time domains.
Discrete-time signals are often referred to as
time series in other fields.
Continuous-time signals are often referred to as
continuous signals. A second important distinction is between discrete-valued and continuous-valued. Particularly in
digital signal processing, a
digital signal may be defined as a sequence of discrete values, typically associated with an underlying continuous-valued physical process. In
digital electronics, digital signals are the continuous-time waveform signals in a digital system, representing a bit-stream. Signals may also be categorized by their spatial distributions as either point source signals (PSSs) or distributed source signals (DSSs). The term
analog signal usually refers to
electrical signals; however, analog signals may use other mediums such as
mechanical,
pneumatic or
hydraulic. 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 electrical fluctuations. The voltage or the current is said to be an
analog of the sound.
Digital signal A digital signal is a signal that is constructed from a discrete set of
waveforms of a physical quantity so as to represent a sequence of
discrete values. A
logic signal is a digital signal with only two possible values, and describes an arbitrary
bit stream. Other types of digital signals can represent
three-valued logic or higher-valued logics. Alternatively, a digital signal may be considered to be the sequence of codes represented by such a physical quantity. The physical quantity may be a variable electric current or voltage, the intensity, phase or
polarization of an
optical or other
electromagnetic field, acoustic pressure, the
magnetization of a
magnetic storage media, etc. Digital signals are present in all
digital electronics, notably computing equipment and
data transmission. With digital signals, system noise, provided it is not too great, will not affect system operation, whereas noise always degrades the operation of
analog signals to some degree. Digital signals often arise via
sampling of analog signals, for example, a continually fluctuating voltage on a line that can be digitized by an
analog-to-digital converter circuit, wherein the circuit will read the voltage level on the line, say, every 50
microseconds and represent each reading with a fixed number of bits. The resulting stream of numbers is stored as digital data on a discrete-time and quantized-amplitude signal.
Computers and other
digital devices are restricted to discrete time.
Energy and power According to the strengths of signals, practical signals can be classified into two categories: energy signals and power signals. Energy signals: Those signals'
energy are equal to a finite positive value, but their average powers are 0; 0 Power signals: Those signals' average
power are equal to a finite
positive value, but their energy are
infinite. P = \lim_{T\rightarrow \infty} \frac{1}{T} \int_{-T/2 }^{T/2} s^2(t)dt
Deterministic and random Deterministic signals are those whose values at any time are predictable and can be calculated by a mathematical equation. Random signals are signals that take on random values at any given time instant and must be modeled
stochastically.
Even and odd An
even signal satisfies the condition x(t) = x(-t) or equivalently if the following equation holds for all t and -t in the domain of x: :x(t) - x(-t) = 0. An odd signal satisfies the condition x(t) = - x(-t) or equivalently if the following equation holds for all t and -t in the domain of x: :x(t) + x(-t) = 0.
Periodic A signal is said to be
periodic if it satisfies the condition: x(t) = x(t + T)\quad \forall t \in [t_0 , t_{max}] or x(n) = x(n + N)\quad \forall n \in [n_0 , n_{max}] Where: T = fundamental time
period, 1/T = f = fundamental
frequency. The same can be applied to N. A periodic signal will repeat for every period.
Time discretization Signals can be classified as
continuous or
discrete time. In the mathematical abstraction, the domain of a continuous-time signal is the set of real numbers (or some interval thereof), whereas the domain of a discrete-time (DT) signal is the set of
integers (or other subsets of real numbers). What these integers represent depends on the nature of the signal; most often, it is time. A continuous-time signal is any
function which is defined at every time
t in an interval, most commonly an infinite interval. A simple source for a discrete-time signal is the
sampling of a continuous signal, approximating the signal by a sequence of its values at particular time instants.
Amplitude quantization If a signal is to be represented as a sequence of digital data, it is impossible to maintain exact precision – each number in the sequence must have a finite number of digits. As a result, the values of such a signal must be
quantized into a
finite set for practical representation. Quantization is the process of converting a continuous analog audio signal to a digital signal with discrete numerical values of integers. == Examples ==