In
signal processing
Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing '' signals'', such as sound, images, and scientific measurements. Signal processing techniques are used to optimize transmissions, ...
, noise is a general term for unwanted (and, in general, unknown) modifications that a
signal 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 order and does not follow an intelligible pattern or combination. Individual ran ...
(
unpredictable) 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.
Noise reduction, the recovery of the original signal from the noise-corrupted one, is a very common goal in the design of signal processing systems, especially
filters. The mathematical limits for noise removal are set by
information theory
Information theory is the scientific study of the quantification, storage, and communication of information. The field was originally established by the works of Harry Nyquist and Ralph Hartley, in the 1920s, and Claude Shannon in the 1940s. ...
.
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, 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
**
Gaussian noise
**
Pink noise or
flicker noise, 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 independent variables and a dependent variable. Standard types of regr ...
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 (a signal produced by a stochastic process). Different colors of noise have significantly ...
**
Cauchy noise
*
Multiplicative noise, 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, 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 sh ...
, e.g. caused by static electricity discharge
*
Transient noise, 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 eng ...
, 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), such as "hiss" or "hum", in audio signals
**
Background noise, due to spurious sounds during signal capture
**
Comfort noise, added to voice communications to fill silent gaps
**
Electromagnetically induced noise, audible noise due to electromagnetic vibrations in systems involving electromagnetic fields
*
Noise (video), such as "snow"
*
Noise (radio), such as "static", in radio transmissions
*
Image noise, affects images, usually digital ones
**
Salt and pepper noise or spike noise, scattered very dark or very light pixels
**
Fixed pattern noise, that is tied to pixel sensors
**
Shadow noise, made visible by increasing brightness or contrast
**
Speckle noise, typical of radar imaging and
interferograms
**
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 beco ...
or "mosquito noise" around edges in JPEG and other formats
*
Noise (electronics) in electrical signals
**
Johnson–Nyquist noise, in semiconductors
**
Quantum noise
**
Quantum 1/f noise, a disputed theory about quantum systems
**
Generation-recombination noise, in semiconductor devices
**
Oscillator phase noise, random fluctuations of the phase of an oscillator
**
Barkhausen effect or Barkhausen noise, in the strength of a ferromagnet
**
Spectral splatter or switch noise, caused by on/off transmitter switching
**
Ground noise, appearing at the ground terminal of audio equipment
*
Synaptic noise, 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
Transcriptional noise is a primary cause of the variability (noise) in gene expression occurring between cells in isogenic populations (see also cellular noise) . A proposed source of transcriptional noise is transcriptional bursting although oth ...
in the transcription of genes to proteins
*
Cosmic noise, in radioastronomy
*
Phonon noise in materials science
*
Internet background noise, 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 Mode partition noise: In an optical communication link, is phase jitter of the signal caused by the combined effects of mode hopping in the optical source and intramodal distortion in the fiber.
Mode hopping causes random wavelength changes wh ...
in optical cables
*
Seismic noise, 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 ...
, often defined by inherent noise level
*
Signal-to-noise ratio (SNR), ratio of noise power to signal power
**
Peak signal-to-noise ratio, 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 bas ...
, the signal-to-noise ratio of a modulated signal
*
Noise power
*
Noise figure
*
Noise-equivalent flux density, a measure of noise in astronomy
*
Noise floor
*
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 le ...
, by how much a signal exceeds the noise level
*
Reference noise, a reference level for electronic noise
*
Noise spectral density, 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, a measure of sensitivity for photodetectors
*
Relative intensity noise, in a laser beam
*
Antenna noise temperature, measure of noise in telecommunications antenna
*
Received noise power
In telecommunications, received noise power is a measure of noise in a receiver. For example, the received noise power might be:
#The calculated or measured noise power, within the bandwidth being used, at the receive end of a circuit, channel, ...
, noise at a telecommunications receiver
*
Circuit noise level, 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, ratio of received carrier power to receiver noise
*
Carrier-to-noise-density ratio,
*
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, pro ...
, a measure of antenna performance
*
Contrast-to-noise ratio, 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
Mastering, a form of audio post production, is the process of preparing and transferring recorded audio from a source containing t ...
, statistical signature of ambient noise for its suppression
*
Equivalent pulse code modulation noise, 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
*
Antenna analyzer or noise bridge, used to measure the efficiency of antennas
*
Noise gate
*
Noise generator, a circuit that produces a random electrical signal
*
Radio noise source used to calibrate radiotelescopes
*
Friis formulas for the noise Friis is a name of Danish origin, meaning Frisian person. It may refer to any of the following people:
* Astrid Friis (1893–1966), Danish historian
* Christen Friis (1581-1639), Danish politician
* Eigil Friis-Christensen, Danish geophysicist ...
in telecommunications
*
Noise-domain reflectometry, uses existing signals to find cable faults
*
See also
*
Anti-information
*
Noise (electronics)
*
Signal-to-noise statistic, a mathematical formula to measure the difference of two values relative to their standard deviations
References
{{reflist
Signal processing