In signal processing, noise is a general term for unwanted (and, in general, unknown) modifications that a signal may suffer during capture, storage, transmission, processing, or conversion.
Sometimes the word is also used to mean signals that are random (unpredictable) and carry no useful information; 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, namely the Nyquist–Shannon sampling theorem.
Signal processing noise can be classified by its statistical properties (sometimes called the "color" of the noise) and by how it modifies the intended signal:
Additive noise, gets added to the intended signalWhite noiseAdditive white Gaussian noisePink noiseBlack noiseGaussian noiseFlicker noise, with 1/f power spectrumBrown noise or Brownian noise, with 1/f2 power spectrumContaminated Gaussian noise, whose PDF is a linear mixture of Gaussian PDFsPower-law noiseCauchy noiseMultiplicative noise, multiplies or modulates the intended signalQuantization error, due to conversion from continuous to discrete valuesPoisson noise, typical of signals that are rates of discrete eventsShot noise, e.g. caused by static electricity dischargeTransient noise, a short pulse followed by decaying oscillationsBurst noise, powerful but only during short intervalsPhase noise, random time shifts in a signalNoise 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 signalsBackground noise, due to spurious sounds during signal captureComfort noise, added to voice communications to fill silent gapsNoise (video), such as "snow"Noise (radio), such as "static", in radio transmissionsImage noise, affects images, usually digital onesSalt and pepper noise or spike noise, scattered very dark or very light pixelsFixed pattern noise, that is tied to pixel sensorsShadow noise, made visible by increasing brightness or contrastSpeckle noise, typical of radar imaging and interferogramsFilm grain in analog photographyCompression artifacts or "mosquito noise" around edges in JPEG and other formatsNoise (electronics) in electrical signalsJohnson–Nyquist noise, in semiconductorsQuantum noiseQuantum 1/f noise, a disputed theory about quantum systemsCoil noise, audible and electronic, caused by vibrating inductors and transformersGeneration-recombination noise, in semiconductor devicesOscillator phase noise, random fluctuations of the phase of an oscillatorBarkhausen effect or Barkhausen noise, in the strength of a ferromagnetSpectral splatter or switch noise, caused by on/off transmitter switchingGround noise, appearing at the ground terminal of audio equipmentSynaptic noise, observed in neuroscienceNeuronal noise, observed in neuroscienceTranscriptional noise in the transcription of genes to proteinsCosmic noise, in radioastronomyPhonon noise in materials scienceInternet background noise, packets sent to unassigned or inactive IP addressesFano noise, in particle detectorsMode partition noise in optical cablesSeismic noise, spurious ground vibrations in seismologyCosmic microwave background, microwave noise left over from the Big BangA 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, often defined by inherent noise levelSignal-to-noise ratio (SNR), ratio of noise power to signal powerPeak signal-to-noise ratio, maximum SNR in a systemSignal to noise ratio (imaging), for imagesCarrier-to-noise ratio, the signal-to-noise ratio of a modulated signalNoise powerNoise figureNoise-equivalent flux density, a measure of noise in astronomyNoise floorNoise margin, by how much a signal exceeds the noise levelReference noise, a reference level for electronic noiseNoise spectral density, noise power per unit of bandwidthNoise temperatureEffective input noise temperatureNoise-equivalent power, a measure of sensitivity for photodetectorsRelative intensity noise, in a laser beamAntenna noise temperature, measure of noise in telecommunications antennaReceived noise power, noise at a telecommunications receiverCircuit noise level, ratio of circuit noise to some reference levelChannel noise level, some measure of noise in a communication channelNoise-equivalent target, intensity of a target when the signal-to-noise level is 1Equivalent noise resistance, a measure of noise based on equivalent resistorCarrier-to-receiver noise density, ratio of received carrier power to receiver noiseCarrier-to-noise-density ratio,Spectral signal-to-noise ratioAntenna gain-to-noise temperature, a measure of antenna performanceContrast-to-noise ratio, a measure of image qualityNoise print, statistical signature of ambient noise for its suppressionEquivalent pulse code modulation noise, measure of noise by comparing to PCM quantization noiseAlmost every technique and device for signal processing has some connection to noise. Some random examples are:
Noise shapingAntenna analyzer or noise bridge, used to measure the efficiency of antennasNoise gateNoise generator, a circuit that produces a random electrical signalRadio noise source used to calibrate radiotelescopesFriis formulas for the noise in telecommunicationsNoise-domain reflectometry, uses existing signals to find cable faultsNoise-immune cavity-enhanced optical heterodyne molecular spectroscopy