signal processing Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing ''signals'', such as audio signal processing, sound, image processing, images, and scientific measurements. Signal processing techniq ...

, noise is a general term for unwanted (and, in general, unknown) modifications that a

signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...

may suffer during capture, storage, transmission, processing, or conversion. Vyacheslav Tuzlukov (2010), ''Signal Processing Noise'', Electrical Engineering and Applied Signal Processing Series, CRC Press. 688 pages. Sometimes the word is also used to mean signals that are

random In common usage, randomness is the apparent or actual lack of pattern or predictability in events. A random sequence of events, symbols or steps often has no :wikt:order, order and does not follow an intelligible pattern or combination. Ind ...

(

unpredictable Unpredictable may refer to: Film and TV * "Unpredictable" (''Eureka''), an episode of ''Eureka'' Music Albums * ''Unpredictable'' (Classified album), 2000 * ''Unpredictable'' (Jamie Foxx album), 2005 * ''Unpredictable'' (Malik B. album), 2015 ...

) and carry no useful

information Information is an abstract concept that refers to that which has the power to inform. At the most fundamental level information pertains to the interpretation of that which may be sensed. Any natural process that is not completely random ...

; even if they are not interfering with other signals or may have been introduced intentionally, as in

comfort noise Comfort noise (or comfort tone) is synthetic background noise used in radio and wireless communications to fill the artificial silence in a transmission resulting from voice activity detection or from the audio clarity of modern digital lines. ...

Noise reduction Noise reduction is the process of removing noise from a signal. Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree. Noise rejection is the ability of a circuit to isolate an und ...

, the recovery of the original signal from the noise-corrupted one, is a very common goal in the design of signal processing systems, especially

filter Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component tha ...

s. The mathematical limits for noise removal are set by

information theory Information theory is the scientific study of the quantification (science), quantification, computer data storage, storage, and telecommunication, communication of information. The field was originally established by the works of Harry Nyquist a ...

Types of noise

Signal processing noise can be classified by its statistical properties (sometimes called the "

color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...

" of the noise) and by how it modifies the intended signal: *

Additive noise Additive white Gaussian noise (AWGN) is a basic noise model used in information theory to mimic the effect of many random processes that occur in nature. The modifiers denote specific characteristics: * ''Additive'' because it is added to any nois ...

, gets added to the intended signal **

White noise In signal processing, white noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. The term is used, with this or similar meanings, in many scientific and technical disciplines, ...

***

Additive white Gaussian noise Additive white Gaussian noise (AWGN) is a basic noise model used in information theory to mimic the effect of many random processes that occur in nature. The modifiers denote specific characteristics: * ''Additive'' because it is added to any nois ...

Black noise In audio engineering, electronics, physics, and many other fields, the color of noise or noise spectrum refers to the power spectrum of a noise signal (a signal produced by a stochastic process). Different colors of noise have significantl ...

Gaussian noise Gaussian noise, named after Carl Friedrich Gauss, is a term from signal processing theory denoting a kind of signal noise that has a probability density function (pdf) equal to that of the normal distribution (which is also known as the Gaussia ...

Pink noise Pink noise or noise is a signal or process with a frequency spectrum such that the power spectral density (power per frequency interval) is inversely proportional to the frequency of the signal. In pink noise, each octave interval (halving ...

flicker noise Flicker noise is a type of electronic noise with a 1/''f'' power spectral density. It is therefore often referred to as 1/''f'' noise or pink noise, though these terms have wider definitions. It occurs in almost all electronic devices and can show ...

, with 1/''f'' power spectrum **

Brownian noise ] In science, Brownian noise, also known as Brown noise or red noise, is the type of signal noise produced by Brownian motion, hence its alternative name of random walk noise. The term "Brown noise" does not come from the color, but after ...

, with 1/''f''² power spectrum **

Contaminated Gaussian In robust statistics, robust regression seeks to overcome some limitations of traditional regression analysis. A regression analysis models the relationship between one or more Dependent and independent variables#Statistics, independent variables ...

noise, whose PDF is a linear mixture of Gaussian PDFs ** Power-law noise ** Cauchy noise *

Multiplicative noise In signal processing, the term multiplicative noise refers to an unwanted random signal that gets multiplied into some relevant signal during capture, transmission, or other processing. An important example is the speckle noise commonly observed ...

, multiplies or modulates the intended signal *

Quantization error Quantization, in mathematics and digital signal processing, is the process of mapping input values from a large set (often a continuous set) to output values in a (countable) smaller set, often with a finite number of elements. Rounding and ...

, due to conversion from continuous to discrete values *

Poisson noise Shot noise or Poisson noise is a type of noise which can be modeled by a Poisson process. In electronics shot noise originates from the discrete nature of electric charge. Shot noise also occurs in photon counting in optical devices, where shot ...

, typical of signals that are rates of discrete events *

Shot noise Shot noise or Poisson noise is a type of noise which can be modeled by a Poisson process. In electronics shot noise originates from the discrete nature of electric charge. Shot noise also occurs in photon counting in optical devices, where shot ...

, e.g. caused by static electricity discharge *

Transient noise Transient noise pulses consist of a relatively short pulse followed by decaying low frequency oscillations. The initial peak is often due to an impulse interference, and the following oscillations are due to resonance on the channel that received t ...

, a short pulse followed by decaying oscillations * Burst noise, powerful but only during short intervals *

Phase noise In signal processing, phase noise is the frequency-domain representation of random fluctuations in the phase of a waveform, corresponding to time-domain deviations from perfect periodicity (jitter). Generally speaking, radio-frequency engineers ...

, random time shifts in a signal

Noise in specific kinds of signals

Noise may arise in signals of interest to various scientific and technical fields, often with specific features: *

Noise (audio) Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...

, such as "hiss" or "hum", in audio signals **

Background noise Background noise or ambient noise is any sound other than the sound being monitored (primary sound). Background noise is a form of noise pollution or interference. Background noise is an important concept in setting noise levels. Background n ...

, due to spurious sounds during signal capture **

Comfort noise Comfort noise (or comfort tone) is synthetic background noise used in radio and wireless communications to fill the artificial silence in a transmission resulting from voice activity detection or from the audio clarity of modern digital lines. ...

, added to voice communications to fill silent gaps ** Electromagnetically induced noise, audible noise due to electromagnetic vibrations in systems involving electromagnetic fields *

Noise (video) Noise, commonly known as static, white noise, static noise, or snow, in analog video and television, is a random dot pixel pattern of static displayed when no transmission signal is obtained by the antenna receiver of television sets and other d ...

, such as "snow" *

Noise (radio) In radio reception, radio noise is unwanted random radio frequency electrical signals, fluctuating voltages, always present in a radio receiver in addition to the desired radio signal. Radio noise near in frequency to the radio signal being receive ...

, such as "static", in radio transmissions *

Image noise Image noise is random variation of brightness or color information in images, and is usually an aspect of electronic noise. It can be produced by the image sensor and circuitry of a Image scanner, scanner or digital camera. Image noise can also ...

, affects images, usually digital ones **

Salt and pepper noise Salt-and-pepper noise, also known as impulse noise, is a form of noise sometimes seen on digital images. This noise can be caused by sharp and sudden disturbances in the image signal. It presents itself as sparsely occurring white and black pixe ...

or spike noise, scattered very dark or very light pixels ** Fixed pattern noise, that is tied to pixel sensors **

Shadow noise {{main, Image noise Shadow noise or luminescence noise is an effect that digital lightening has on an image, specifically on the darker, or shadowed areas of the image. Brightening the image, especially in underexposed photos, brings out the "shad ...

, made visible by increasing brightness or contrast **

Speckle noise Speckle, speckle pattern, or speckle noise is a granular noise texture degrading the quality as a consequence of interference among wavefronts in coherent imaging systems, such as radar, synthetic aperture radar (SAR), medical ultrasound and op ...

, typical of radar imaging and

interferogram In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...

s **

Film grain Film grain or granularity is the random optical texture of processed photographic film due to the presence of small particles of a metallic silver, or dye clouds, developed from silver halide that have received enough photons. While film grain ...

in analog photography **

Compression artifacts A compression artifact (or artefact) is a noticeable distortion of media (including images, audio, and video) caused by the application of lossy compression. Lossy data compression involves discarding some of the media's data so that it bec ...

or "mosquito noise" around edges in JPEG and other formats *

Noise (electronics) In electronics, noise is an unwanted disturbance in an electrical signal. Noise generated by electronic devices varies greatly as it is produced by several different effects. In particular, noise is inherent in physics, and central to the ...

in electrical signals **

Johnson–Nyquist noise Johnson–Nyquist noise (thermal noise, Johnson noise, or Nyquist noise) is the electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor at equilibrium, which happens rega ...

, in semiconductors **

Quantum noise Quantum noise is noise arising from the indeterminate state of matter in accordance with fundamental principles of quantum mechanics, specifically the uncertainty principle and via zero-point energy fluctuations. Quantum noise is due to the appa ...

** Quantum 1/f noise, a disputed theory about quantum systems ** Generation-recombination noise, in semiconductor devices **

Oscillator phase noise Oscillators produce various levels of phase noise, or variations from perfect periodicity. Viewed as an additive noise, phase noise increases at frequencies close to the oscillation frequency or its harmonics. With the additive noise being clos ...

, random fluctuations of the phase of an oscillator **

Barkhausen effect The Barkhausen effect is a name given to the noise in the magnetic output of a ferromagnet when the magnetizing force applied to it is changed. Discovered by German physicist Heinrich Barkhausen in 1919, it is caused by rapid changes of size o ...

or Barkhausen noise, in the strength of a ferromagnet **

Spectral splatter {{unreferenced, date=February 2008 In radio electronics or acoustics, spectral splatter (also called ''switch noise'') refers to spurious emissions that result from an abrupt change in the transmitted signal, usually when transmission is started ...

or switch noise, caused by on/off transmitter switching **

Ground noise In an electrical system, a ground loop or earth loop occurs when two points of a circuit are intended to have the same ground reference potential but instead have a different potential between them. This is typically caused when enough curren ...

, appearing at the ground terminal of audio equipment *

Synaptic noise Synaptic noise refers to the constant bombardment of synaptic activity in neurons. This occurs in the background of a cell when potentials are produced without the nerve stimulation of an action potential, and are due to the inherently random natur ...

, observed in neuroscience * Neuronal noise, observed in neuroscience * Transcriptional noise in the transcription of genes to proteins *

Cosmic noise Cosmic noise, also known as galactic radio noise, is not actually sound, but a physical phenomenon derived from outside of the Earth's atmosphere. It can be detected through a radio receiver, which is an electronic device that receives radio waves ...

, in radioastronomy *

Phonon noise Phonon noise, also known as thermal fluctuation noise, arises from the random exchange of energy between a thermal mass and its surrounding environment. This energy is quantized in the form of phonons. Each phonon has an energy of order k_\textT, w ...

in materials science *

Internet background noise Internet background noise (IBN, also known as Internet background radiation) consists of data packets on the Internet which are addressed to IP addresses or ports where there is no network device set up to receive them. These packets often contain ...

, packets sent to unassigned or inactive IP addresses * Fano noise, in particle detectors * Mode partition noise in optical cables *

Seismic noise In geophysics, geology, civil engineering, and related disciplines, seismic noise is a generic name for a relatively persistent vibration of the ground, due to a multitude of causes, that is often a non-interpretable or unwanted component of signal ...

, spurious ground vibrations in seismology *

Cosmic microwave background In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...

, microwave noise left over from the Big Bang

Measures of noise in signals

A long list of noise measures have been defined to measure noise in signal processing: in absolute terms, relative to some standard noise level, or relative to the desired signal level. They include: *

Dynamic range Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base-1 ...

, often defined by inherent noise level *

Signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...

(SNR), ratio of noise power to signal power **

Peak signal-to-noise ratio Peak signal-to-noise ratio (PSNR) is an engineering term for the ratio between the maximum possible power of a signal and the power of corrupting noise that affects the fidelity of its representation. Because many signals have a very wide dynamic ...

, maximum SNR in a system ** Signal to noise ratio (imaging), for images ** Carrier-to-noise ratio, the signal-to-noise ratio of a modulated signal *

Noise power In telecommunication, the term noise power has the following meanings: # The measured total noise in a given bandwidth at the input or output of a device when the signal is not present; the integral of noise spectral density over the bandwidth # T ...

Noise figure Noise figure (NF) and noise factor (''F'') are figures of merit that indicate degradation of the signal-to-noise ratio (SNR) that is caused by components in a signal chain. These figures of merit are used to evaluate the performance of an amplifier ...

Noise-equivalent flux density In optics the noise-equivalent flux density (NEFD) or noise-equivalent irradiance (NEI) of a system is the level of flux density required to be equivalent to the noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearin ...

, a measure of noise in astronomy *

Noise floor In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system, where noise is defined as any signal other than the one being monitored. In radio com ...

Noise margin In electrical engineering, noise margin is the maximum voltage amplitude of extraneous signal (electrical engineering), signal that can be algebraically added to the noise-free worst-case input level without causing the output voltage to deviate fr ...

, by how much a signal exceeds the noise level *

Reference noise In telecommunication, reference noise is the magnitude of circuit noise chosen as a reference for measurement. Many different levels with a number of different weightings are in current use, and care must be taken to ensure that the proper param ...

, a reference level for electronic noise *

Noise spectral density In communications, noise spectral density (NSD), noise power density, noise power spectral density, or simply noise density (''N''0) is the power spectral density of noise or the noise power per unit of bandwidth. It has dimension of power over fre ...

, noise power per unit of bandwidth *

Noise temperature In electronics, noise temperature is one way of expressing the level of available noise power introduced by a component or source. (This is to be distinguished from Temperature Noise in Thermodynamics or Principal Interferrometric Analysis Over C ...

* Effective input noise temperature *

Noise-equivalent power Noise-equivalent power (NEP) is a measure of the sensitivity of a photodetector or detector system. It is defined as the signal power that gives a signal-to-noise ratio of one in a one hertz output bandwidth. An output bandwidth of one hertz is equi ...

, a measure of sensitivity for photodetectors *

Relative intensity noise Relative intensity noise (RIN), describes the instability in the power (physics), power level of a laser. The noise term is important to describe lasers used in fiber-optic communication and LIDAR remote sensing. Relative intensity noise can be gen ...

, in a laser beam *

Antenna noise temperature In radio frequency (RF) applications such as radio, radar and telecommunications, noise temperature of an antenna is a measure of the noise power density contributed by the antenna to the overall RF receiver system. It is defined as "The temperatu ...

, measure of noise in telecommunications antenna * Received noise power, noise at a telecommunications receiver *

Circuit noise level At any point in a transmission system, the ratio of the circuit noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both a ...

, ratio of circuit noise to some reference level * Channel noise level, some measure of noise in a communication channel * Noise-equivalent target, intensity of a target when the signal-to-noise level is 1 * Equivalent noise resistance, a measure of noise based on equivalent resistor *

Carrier-to-receiver noise density In telecommunications, the carrier-to-noise ratio, often written CNR or ''C/N'', is the signal-to-noise ratio (SNR) of a modulated signal. The term is used to distinguish the CNR of the radio frequency passband signal from the SNR of an analog b ...

, ratio of received carrier power to receiver noise *

Carrier-to-noise-density ratio In telecommunications, the carrier-to-noise ratio, often written CNR or ''C/N'', is the signal-to-noise ratio (SNR) of a modulated signal. The term is used to distinguish the CNR of the radio frequency passband signal from the SNR of an analog b ...

, * Spectral signal-to-noise ratio *

Antenna gain-to-noise temperature AMC-18 is a geostationary Lockheed Martin A2100A communications satellite owned by SES Americom. It was launched on 8 December 2006 from Centre Spatial Guyanais aboard an Ariane 5 ECA launch vehicle and is situated at 83° West longitude, p ...

, a measure of antenna performance *

Contrast-to-noise ratio Contrast-to-noise ratio (CNR) is a measure used to determine image quality. CNR is similar to the metric signal-to-noise ratio (SNR), but subtracts a term before taking the ratio. This is important when there is a significant bias in an image, su ...

, a measure of image quality *

Noise print A noise print is part of a technique used in noise reduction. A noise print is commonly used in audio mastering to help reduce the effects of unwanted noise from a piece of audio. In this case, the noise print would be a recording of the ambient no ...

, statistical signature of ambient noise for its suppression *

Equivalent pulse code modulation noise In telecommunication, equivalent pulse code modulation (PCM) noise is the amount of noise power on a frequency-division multiplexing (FDM) or wire communication channel necessary to approximate the same judgment of speech quality created by quanti ...

, measure of noise by comparing to PCM quantization noise

Technology for noise in signals

Almost every technique and device for signal processing has some connection to noise. Some random examples are: *

Noise shaping Noise shaping is a technique typically used in digital audio, image, and video processing, usually in combination with dithering, as part of the process of quantization or bit-depth reduction of a digital signal. Its purpose is to increase the ap ...

Antenna analyzer An antenna analyzer or in British aerial analyser (also known as a noise bridge, RX bridge, SWR analyzer, or RF analyzer) is a device used for measuring the input impedance of antenna systems in radio electronics applications. In radio communica ...

or noise bridge, used to measure the efficiency of antennas *

Noise gate A noise gate or gate is an electronic device or software that is used to control the volume of an audio signal. Comparable to a compressor, which attenuates signals ''above'' a threshold, such as loud attacks from the start of musical notes, no ...

* Noise generator, a circuit that produces a random electrical signal *

Radio noise source A radio noise source is a device that emits radio waves at a certain frequency, used to calibrate radio telescopes such that received data may be compared to a known value, as well as to find the focal point of a telescope soon after construction, ...

used to calibrate radiotelescopes * Friis formulas for the noise in telecommunications *

Noise-domain reflectometry Noise-domain reflectometry is a type of reflectometry where the reflectometer exploits existing data signals on wiring and does not have to generate any signals itself. Noise-domain reflectometry, like time-domain reflectometry, time-domain and Spr ...

, uses existing signals to find cable faults * Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy

References

{{reflist Signal processing

Types of noise

Noise in specific kinds of signals

Measures of noise in signals

Technology for noise in signals

See also

References