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, Scalar potential, potential fields, Seismic tomograph ...

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

signal A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology. In ...

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 definite pattern or predictability in information. A random sequence of events, symbols or steps often has no order and does not follow an intelligible pattern or combination. ...

( unpredictable) and carry no useful

information Information is an Abstraction, abstract concept that refers to something which has the power Communication, to inform. At the most fundamental level, it pertains to the Interpretation (philosophy), interpretation (perhaps Interpretation (log ...

; 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 u ...

, 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 Filtration is a physical process that separates solid matter and fluid from a mixture. Filter, filtering, filters or filtration may also refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Fil ...

s. The mathematical limits for noise removal are set by

information theory Information theory is the mathematical study of the quantification (science), quantification, Data storage, storage, and telecommunications, communication of information. The field was established and formalized by Claude Shannon in the 1940s, ...

Types of noise

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

color Color (or colour in English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is the visual perception based on the electromagnetic spectrum. Though co ...

" 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, i ...

***

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 **

Gaussian noise Carl Friedrich Gauss (1777–1855) is the eponym of all of the topics listed below. There are over 100 topics all named after this German mathematician and scientist, all in the fields of mathematics, physics, and astronomy. The English eponymo ...

Pink noise Pink noise, noise, fractional noise or fractal noise is a signal (information theory), signal or process with a frequency spectrum such that the power spectral density (power per frequency interval) is inversely proportional to the frequenc ...

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 brown, the color, but after ...

, with 1/''f''² power spectrum ** Contaminated Gaussian noise, whose PDF is a linear mixture of Gaussian PDFs **

Power-law 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 processing), noise signal (a signal produced by a stochastic process). Different colors of ...

** 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. Multiplicative noise is a type of signal-dependent noise whe ...

, 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 sh ...

, 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 s ...

, 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 Burst noise is a type of electronic noise that occurs in semiconductors and ultra-thin gate oxide films. It is also called random telegraph noise (RTN), popcorn noise, impulse noise, bi-stable noise, or random telegraph signal (RTS) noise. It c ...

, 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 enginee ...

, 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 sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibratio ...

, 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 no ...

, 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, CRTs and television, is a random dot pixel pattern of static displayed when no transmission signal is obtained by the antenna receiver of television sets ...

, such as "snow" *

Noise (radio) In radio reception, radio noise (commonly referred to as radio static) is unwanted random radio frequency electrical signals, fluctuating voltages, always present in a radio receiver in addition to the desired radio signal. Radio noise is a comb ...

, such as "static", in radio transmissions *

Image noise Image noise is random variation of brightness or color information in images. It can originate in film grain and in the unavoidable shot noise of an ideal photon detector. In digital photography is usually an aspect of electronic noise, produ ...

, 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. For black-and-white or grayscale images, it presents as sparsely occurring white and black pixels, giving the appearance of an image sprinkl ...

or spike noise, scattered very dark or very light pixels ** Fixed pattern noise, tied to pixel sensors ** Shadow noise, made visible by increasing brightness or contrast **

Speckle noise Speckle, speckle pattern, or speckle noise designates the granular structure observed in coherent light, resulting from random interference. Speckle patterns are used in a wide range of metrology techniques, as they generally allow high sensitivi ...

, typical of radar imaging and

interferogram In physics, interference is a phenomenon in which two coherent waves are combined by adding their intensities or displacements with due consideration for their phase difference. The resultant wave may have greater amplitude (constructive inte ...

s **

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

in analog photography **

Compression artifacts A compression artifact (or artefact) is a noticeable distortion of media (including Image, images, Sound recording, audio, and video) caused by the application of lossy compression. Lossy data compression involves discarding some of the medi ...

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 thermo ...

in electrical signals **

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

, 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 in the ...

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

Spectral splatter 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 stopped. For example, a device ...

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 current ...

, 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 natu ...

, observed in neuroscience *

Neuronal noise Neuronal noise or neural noise refers to the random intrinsic electrical fluctuations within neuronal networks. These fluctuations are not associated with encoding a response to internal or external stimuli and can be from one to two orders of mag ...

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

Cosmic noise Cosmic noise, also known as galactic radio noise, is radio-frequency electromagnetic radiation from sources outside of the Earth's atmosphere. Its characteristics are comparable to those of thermal noise. Cosmic noise occurs at frequencies above a ...

, 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, by analogy with natural background radiation) consists of data packets on the Internet addressed to IP addresses or ports where there is no network device set up to rec ...

, packets sent to unassigned or inactive IP addresses *

Fano noise Fano noise is a fluctuation of an electric charge obtained in a detector (in spite of constant value of the measured quantity, which is usually an energy), arising from processes in the detector. It was first described by Ugo Fano in 1947, as a fl ...

, 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 signa ...

, spurious ground vibrations in seismology *

Cosmic microwave background The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...

, microwave noise left over from the Big Bang

Measures of noise in signals

A long list of noise measures has 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 Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' " power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics), the study of forces and their effect on motion Brands and ent ...

, 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 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 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 ...

, the signal-to-noise ratio of a modulated signal *

Noise power In telecommunications, 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 # ...

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 amplifie ...

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 present in the system. It is a measure used by astronomers in determining the ac ...

, 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 that can be algebraically added to the noise-free worst-case input level without causing the output voltage to deviate from the allowable logic voltage l ...

, by how much a signal exceeds the noise level *

Reference noise A reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''nam ...

, 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 f ...

, 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. The power spectral density of the noise is expressed in terms of the temperature (in kelvins) that would produce ...

Effective input noise temperature In telecommunications, effective input noise temperature is the source noise temperature in a two-port network or amplifier that will result in the same output noise power, when connected to a noise-free network or amplifier, as that of the actual n ...

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 equ ...

, a measure of sensitivity for photodetectors * Relative intensity noise, in a laser beam *

Antenna noise temperature In radio frequency (RF) applications such as radio, radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It ...

, 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 sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, the ...

, 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 an 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 ...

, 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, 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, s ...

, 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 noi ...

, statistical signature of ambient noise for its suppression *

Equivalent pulse code modulation noise In telecommunications, 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 quantiz ...

, 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 processing, image, and video processing, usually in combination with dithering, as part of the process of Quantization (signal processing), quantization or Audio bit depth, bit-dep ...

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 communic ...

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

Noise gate A noise gate or simply gate is an electronic device or software that is used to control the amplitude, volume of an audio signal. Comparable to a limiter, which attenuates signals ''above'' a threshold, such as loud attacks from the start of mu ...

Noise generator A noise generator is a circuit that produces electrical noise (i.e., a random signal). Noise generators are used to test signals for measuring noise figure, frequency response, and other parameters. Noise generators are also used for Random numbe ...

, 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 telescope A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio ...

used to calibrate radiotelescopes *

Friis formulas for noise Friis formula or Friis's formula (sometimes Friis' formula), named after Danish-American electrical engineer Harald T. Friis, is either of two formulas used in telecommunications engineering to calculate the signal-to-noise ratio of a multistag ...

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