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Encyclopedia > Noise reduction

Noise reduction is the process of removing noise from a signal. Noise reduction techniques are conceptually very similar regardless of the signal being processed, however a priori knowledge of the characteristics of an expected signal can mean the implementations of these techniques vary greatly depending on the type of signal. For the Irish mythological figure, see Naoise. ... In information theory, a signal is the sequence of states of a communications channel that encodes a message. ... The terms a priori and a posteriori are used in philosophy to distinguish between two different types of propositional knowledge. ...


All recording devices, both analogue or digital, have traits which make them susceptible to noise. Noise can be random or white noise with no coherence or coherent noise introduced by the devices mechanism or processing algorithms. Wikipedia does not yet have an article with this exact name. ... A digital system is one that uses discrete values (often electrical voltages), especially those representable as binary numbers, or non-numeric symbols such as letters or icons, for input, processing, transmission, storage, or display, rather than a continuous spectrum of values (ie, as in an analog system). ... White noise spectrum White noise( ) is a random signal (or process) with a flat power spectral density. ... In mathematics, computing, linguistics, and related disciplines, an algorithm is a finite list of well-defined instructions for accomplishing some task that, given an initial state, will terminate in a defined end-state. ...


In electronic recording devices a major form of noise is hiss caused by random electrons, which are heavily influenced by heat, stray from their designated path. These stray electrons influence the voltage of the output signal and thus create detectable noise. An electronic circuit is an electrical circuit that also contains active electronic devices such as transistors or vacuum tubes. ... e- redirects here. ... International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ...


In the case of photographic film and magnetic tape, noise (both visual and aural) is introduced due to the grain structure of the medium. In photographic film, the size of the grains in the film (more senstive film having larger sized grains) introduces noise. In magnetic tape the larger the grains of the magnetic particles, usually ferric oxide or magnetite. The larger the grains, the more prone the medium is to noise introduced by the grain. This article or section does not cite its references or sources. ... Compact audio cassette Magnetic tape is a non-volatile storage medium consisting of a magnetic coating on a thin plastic strip. ... Iron(III) oxide - also known as ferric oxide, red iron oxide, synthetic maghemite, rouge,or rust - is one of several oxide compounds of iron, and is most notable for its ferromagnetic properties. ... // Headline text Magnetite is a ferrimagnetic mineral form of iron(II,III) oxide, with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. ...


To compensate for this, larger areas of film or magnetic tape may be used to lower the noise level to an acceptable point.

Contents

Audio noise reduction

When using analog recording technology, sound recordings exhibit a type of noise known as tape hiss. This is related to the particle size and texture used in the magnetic emulsion that is sprayed on the recording media, and also to the relative tape velocity across the tape heads. An analog or analogue signal is any continuously variable signal. ... Tape hiss is the high frequency noise present on analogue magnetic tape recordings caused by the size of the magnetic particles used to make the tape. ... A tape head is a type of transducer used in tape recorders to convert electrical signals to magnetic fluctuations and vice versa. ...


While there are dozens of different kinds of noise reduction, the first widely used audio noise reduction technique was developed by Ray Dolby in 1966. Intended for professional use, Dolby Type A was an encode/decode system in which the amplitude of frequencies in four bands was increased during recording (encoding), then decreased proportionately during playback (decoding). The Dolby B system (developed in conjunction with Henry Kloss) was a single band system designed for consumer products. In particular, when recording quiet parts of an audio signal, the frequencies above 1 kHz would be boosted. This had the effect of increasing the signal to noise ratio on tape up to 10dB depending on the initial signal volume. When it was played back, the decoder reversed the process, in effect reducing the noise level by up to 10dB. The Dolby B system, while not as effective as Dolby A, had the advantage of remaining listenable on playback systems without a decoder. ` Dolby (left) is inducted into the NIHF Ray Dolby (born January 18, 1933) is the American inventor of the noise reduction system known as Dolby NR. He is the founder and chairman of Dolby Laboratories, and a billionaire. ... Henry Kloss (1929, Altoona, PA–January 31, 2002, Cambridge, MA) was a prominent audio engineer and businessman who helped advance high fidelity loudspeaker and radio receiver technology beginning in the 1950s. ...

Dbx was the competing analog noise reduction system developed by dbx laboratories. It used a root-mean-squared (RMS) encode/decode algorithm with the noise-prone high frequencies boosted, and the entire signal fed through a 2:1 compander. Dbx operated across the entire audible bandwidth and unlike Dolby B was unusable as an open ended system. However it could achieve up to 30 dB of noise reduction. Since Analog video recordings use frequency modulation for the luminance part (composite video signal in direct colour systems), which keeps the tape at saturation level, audio style noise reduction is unnecessary. Dolby NR is a noise reduction system developed by Dolby Laboratories for use in analogue magnetic tape recording. ... The logo represents both the company and its noise reduction system dbx is a noise reduction system for analog tape recording, North American TV broadcasting, and, less commonly, vinyl LPs. ... dbx, Inc. ... A waveform before and after the compression stage of companding In telecommunication, signal processing, and thermodynamics, companding (occasionally called compansion) is a method of reducing the effects of a channel with limited dynamic range. ... Video is the technology of capturing, recording, processing, transmitting, and reconstructing moving pictures, typically using celluloid film, electronic signals, or digital media, primarily for viewing on television or as video clips on computer monitors. ...


Modern digital sound (and picture) recordings no longer need to worry about tape hiss either so analog style noise reduction systems are not necessary. However an interesting twist is that dither systems actually add noise to a signal to improve its quality. Dither is a form of noise, or erroneous signal or data which is deliberately added to sample data for the purpose of minimizing quantization error. ...


Image noise reduction

Images taken with both digital cameras and conventional film cameras will pick up noise from a variety of sources. Many further uses of these images require that the noise will be (partially) removed - for aesthetic purposes as in artistic work or marketing, or for practical purposes such as computer vision. This article or section does not cite any references or sources. ... Photographic film a sheet of plastic (polyester, celluloid (nitrocellulose) or cellulose acetate) coated with an emulsion containing light-sensitive silver halide salts (bonded by gelatin) with variable crystal sizes that determine the sensitivity or resolution of the film. ... The Parthenons facade showing an interpretation of golden rectangles in its proportions. ... The Bath, a painting by Mary Cassatt (1844-1926). ... Wikibooks has more about this subject: Marketing Look up marketing in Wiktionary, the free dictionary. ... Computer vision is the science and technology of machines that see. ...


Types of image noise

  • In salt and pepper noise (also known as random noise or independent noise), pixels in the image are vastly different in color from their surrounding pixels. The defining characteristic is that the color of a noisy pixel bears no relation to the color of surrounding pixels. Generally this type of noise will only affect a small number of image pixels. When viewed, the image contains dark and white dots, hence the term salt and pepper noise. Typical sources include flecks of dust on the lens or inside the camera, or with digital cameras, faulty CCD elements.
  • In Gaussian noise (dependent noise), an amount of noise is added to every part of the picture. Each pixel in the image will be changed from its original value by a (usually) small amount. Taking a plot of the amount of distortion of a pixel against the frequency with which it occurs produces a Gaussian distribution of noise.

Salt and pepper noise is a form of noise typically seen on images. ... In science, and especially in physics and telecommunication, noise is fluctuations in and the addition of external factors to the stream of target information (signal) being received at a detector. ... This example shows an image with a portion greatly enlarged, in which the individual pixels are rendered as little squares and can easily be seen. ... Photographic lens One of Canons most popular wide angle lenses - 17-40 f/4 L The zoom lens of the Canon Elph A photographic lens (or more correctly, objective) is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images... A specially developed CCD used for ultraviolet imaging in a wire bonded package. ... Gaussian noise is noise that has a probability density function (pdf) of the normal distribution (a. ... The normal distribution, also called Gaussian distribution by scientists (named after Carl Friedrich Gauss due to his rigorous application of the distribution to astronomical data (Havil, 2003)), is a continuous probability distribution of great importance in many fields. ...

Removing image noise

Gaussian filters

One method to remove noise is by convolving the original image with a mask. The Gaussian mask comprises elements determined by a Gaussian function. It gives the image a blurred appearance if the standard deviation of the mask is high, and has the effect of smearing out the value of a single pixel over an area of the image. This brings the value of each pixel into closer harmony with the value of its neighbors. Gaussian filtering works relatively well, but the blurring of edges can cause problems, particularly if the output is being fed into edge detection algorithms for computer vision applications. In mathematics and, in particular, functional analysis, convolution is a mathematical operator which takes two functions f and g and produces a third function that in a sense represents the amount of overlap between f and a reversed and translated version of g. ... Gaussian curves parametrised by expected value and variance (see normal distribution) A Gaussian function (named after Carl Friedrich Gauss) is a function of the form: for some real constants a > 0, b, and c. ... The goal of edge detection is to mark the points in a digital image at which the luminous intensity changes sharply. ... Computer vision is the science and technology of machines that see. ...


Averaging

Averaging is a degenerate case of Gaussian filtering, where the function defining the mask values has an infinite standard deviation.


Non-Linear filters

A median filter is an example of a non-linear filter and, if properly designed, is very good at preserving image detail. To run a median filter: In image processing it is usually necessary to perform high degree of noise reduction in an image before performing higher-level processing steps, such as edge detection. ...

  1. consider each pixel in the image
  2. sort the neighbouring pixels into order based upon their intensities
  3. replace the original value of the pixel with the median value from the list

This type of filter is very good at removing salt and pepper noise from an image, and also causes very little blurring of edges, and hence is often used in computer vision applications. In probability theory and statistics, a median is a number dividing the higher half of a sample, a population, or a probability distribution from the lower half. ...


Software programs

There are several software programs available designed to remove image noise, such as Noise Ninja, Neat Image and GREYCstoration. Some raster graphics editors, such as PhotoImpact, Adobe Photoshop and Helicon Filter, also feature noise reduction abilities. A screenshot from the KDE raster graphics editor KolourPaint A screenshot from the GIMP raster graphics editor A raster graphics editor is a computer program that allows users to paint and edit pictures interactively on the computer screen and save them in one of many popular bitmap or raster formats... Ulead Systems (TSEC:2487) was a Taiwanese computer software company headquartered in Neihu district in Taipei, Taiwan. ... Adobe Photoshop, or simply Photoshop, is a graphics editor developed and published by Adobe Systems. ... Helicon Filter, often referred to as Helicon or as Filter, is a proprietary commercial and freeware photo editing software program for Microsoft Windows, similar to such programs as Adobe Photoshop and the open source GIMP, developed and published by Helicon Soft Ltd. ...


See also

A dark frame is a photography technique to identify noise in a CCD imaging device by recording without exposing the CCD, usually by leaving the shutter closed. ... For the Irish mythological figure, see Naoise. ... In information theory, a signal is the sequence of states of a communications channel that encodes a message. ... Signal processing is the processing, amplification and interpretation of signals, and deals with the analysis and manipulation of signals. ... Noise-cancelling headphones reduce unwanted ambient sounds (i. ... Digital image processing is the use of computer algorithms to perform image processing on digital images. ... A noise print is a technique in noise reduction. ... There are two types of sound masking systems - in-plenum systems and direct field systems. ... It has been suggested that Acoustic transmission be merged into this article or section. ... It has been suggested that Sound masking system be merged into this article or section. ...

External links


  Results from FactBites:
 
Noise Reduction: Digital Imaging: Glossary: Learn: Digital Photography Review (511 words)
The key to noise reduction is to reduce or eliminate the noise without deteriorating other aspects of the image.
The noise in the blue sky of the original is very visible in the red channel.
Bad noise reduction methods replace the noise by a wavy pattern in uniform surfaces, visible in the 1X crop.
Noise Reduction (621 words)
All noise has a mechanical origin, which is to say it is the result of waves of pressure transmitted through air as the result of the mechanical movement of some object.
Another source of noise is inherent in a motor’s relationship to both the mechanical and electrical effects of spinning parts moving through the air gap.
Noise inherent in a motor generally cannot be "cured" by the motor installer.
  More results at FactBites »

 
 

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