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WD_437/ 2008
(  Satoshi Kinoshita )
| Series: | Works on paper: Drawings 5 | | Medium: | oilstick on paper | | Size (inches): | 40.2 x 25.2 | | Size (mm): | 1020 x 640 | | Catalog #: | WD_0437 | | Description: | Signed, date and copyright in pencil on the reverse.
Colors of noise -
Even though noise is a random signal, it can have characteristic statistical properties. Spectral density (power distribution in the frequency spectrum) is such a property, which can be used to distinguish different types of noise. This classification by spectral density is given "color" terminology, with different types named after different colors, and is common in different disciplines where noise is an important factor (like acoustics, electrical engineering, and physics).
Many of these definitions assume a signal with components at all frequencies, with a spectral density per unit of bandwidth proportional to 1/fβ. For instance, white noise is flat, with β = 0, while brown has β = 2.
Technical definitions:
The color names for these different types of sounds are derived from a loose analogy between the spectrum of frequencies of sound wave present in the sound (as shown in the blue diagrams) and the equivalent spectrum of light wave frequencies. That is, if the sound wave pattern of "blue noise" were translated into light waves, the resulting light would be blue, and so on.
White noise
White noise is a signal (or process), named by analogy to white light, with equal energy per cycle (hertz). This produces a flat frequency spectrum in linear space. In other words, the signal has equal power in any band of a given bandwidth (power spectral density). For example, the range of frequencies between 40 Hz and 60 Hz contains the same amount of power as the range between 4000 Hz and 4020 Hz has. An infinite-bandwidth white noise signal is purely a theoretical construct. By having power at all frequencies, the total power of such a signal would be infinite. In practice, a signal is "white" if it has a flat spectrum over a defined frequency band.
Pink (or 1/f) noise
The frequency spectrum of pink noise is flat in logarithmic space; it has equal power in bands that are proportionally wide. This means that pink noise would have equal power in the frequency range from 40 to 60 Hz as in the band from 4000 to 6000 Hz. Since humans hear in such a proportional space, where a doubling of frequency is perceived the same regardless of actual frequency (40–60 Hz is heard as the same interval and distance as 4000–6000 Hz), every octave contains the same amount of energy and thus pink noise is often used as a reference signal in audio engineering. That is, the human auditory system perceives approximately equal magnitude on all frequencies. The power density, compared with white noise, decreases by 3 dB per octave (density proportional to 1/f). For this reason, pink noise is often called 1/f noise.
Red (or Brown) noise
Red noise, also called Brown noise or Brownian noise, is similar to pink noise, but with a power density decrease of 6 dB per octave with increasing frequency[1] (density proportional to 1/f2) over a frequency range which does not include DC (in a general sense, does not include a constant component, or value at zero frequency). It can be generated by an algorithm which simulates Brownian motion or by integrating white noise. "Brown" noise is not named for a power spectrum that suggests the color brown; rather, the name is a corruption of Brownian motion. (Nor is it to be confused with the so-called "brown note.") "Red noise" describes the shape of the power spectrum, with pink being between red and white. Also known as "random walk" or "drunkard's walk" noise.
Blue (or azure) noise
Blue noise's power density increases 3 dB per octave with increasing frequency (density proportional to f) over a finite frequency range.[2] In computer graphics, the term "blue noise" is sometimes used more loosely as any noise with minimal low frequency components and no concentrated spikes in energy. This can be good noise for dithering (Mitchell, 1987); retinal cells are arranged in a blue-noise-like pattern for this reason (Yellot, 1983).
Purple (or violet) noise
Purple noise's power density increases 6 dB per octave with increasing frequency[3] (density proportional to f2) over a finite frequency range. It is also known as differentiated white noise or violet noise.
Grey noise
Grey noise is random noise[specify] subjected to a psychoacoustic equal loudness curve (such as an inverted A-weighting curve) over a given range of frequencies, giving the listener the perception that it is equally loud at all frequencies. This is in contrast to white noise, random noise which is in fact equally loud at all frequencies but not perceived as such due to the natural bias of the human ear.[dubious – discuss]
Various noise models are employed in analysis, many of which fall under the above categories. AR noise or "autoregressive noise" is such a model, and generates simple examples of the above noise types, and more.
The Federal Standard 1037C Telecommunications Glossary defines white, pink, blue, and black.
Others:
There are also many "less official" colors, usually with multiple definitions
Red noise
1. A synonym for Brownian noise, as above[4]
2. A synonym for pink noise, as above[5]
3. "Oceanic ambient noise (that is, noise from distant sources) is often described as "red" due to the selective absorption of higher frequencies by the ocean." (Common definition within the oceanographic field, contributed to a newsgroup by P.J. Rovero in 1996.)[6]
Orange noise
"Orange noise is quasi-stationary noise with a finite power spectrum with a finite number of small bands of zero energy dispersed throughout a continuous spectrum. These bands of zero energy are centered about the frequencies of musical notes in whatever scale is of interest. Since all in-tune musical notes are eliminated, the remaining spectrum could be said to consist of sour, citrus, or "orange" notes."[7]
Mellow Yellow noise
Gaussian noise that has been band-limited between 2827-3159 Hz. Typically found in a channels that carry biphase-encoded signals.[citation needed]
Green noise
1. "Green noise is supposedly the background noise of the world. A really long term power spectrum averaged over several outdoor sites. Rather like pink noise with a hump added around 500 Hz."[8]
2. The mid-frequency component of white noise, used in halftone dithering[9]
3. Bounded Brownian noise
Black noise
Black noise, or silent noise, has several different definitions:
1. Silence
2. Noise with a 1/fβ spectrum, where β > 2 (Manfred Schroeder, "Fractals, chaos, power laws"). Used in modeling various environmental processes. Is said to be a characteristic of "natural and unnatural catastrophes like floods, droughts, bear markets, and various outrageous outages, such as those of electrical power." Further, "because of their black spectra, such disasters often come in clusters."[10]
3. Noise that has a frequency spectrum of predominantly zero power level over all frequencies except for a few narrow bands or spikes. Note: An example of black noise in a facsimile transmission system is the spectrum that might be obtained when scanning a black area in which there are a few random white spots. Thus, in the time domain, a few random pulses occur while scanning.[11]
4. "The output of an active noise control system which cancels an existing noise, leaving the local environment noise free. The comic book character Iron Man used to have a "black light beam" that could darken a room like this, and popular sci-fi has a tendency to portray active noise control in this light."[12] The Batman Beyond supervillian Shriek also had a weapon like this, which effectively blocked out all noise.
5. "As seen in the sales literature for an ultrasonic vermin repeller, black noise with a power density that is constant for a finite frequency range above 20 kHz. More accurately, ultrasonic white noise. This black noise is like the so-called black light with frequencies too high to be sensed, but still capable of affecting the environment."[13]
Noisy white
In telecommunication, the term noisy white has the following meanings:
1. In facsimile or display systems, such as television, a nonuniformity in the white area of the image, i.e., document or picture, caused by the presence of noise in the received signal.
2. A signal or signal level that is supposed to represent a white area on the object, but has a noise content sufficient to cause the creation of noticeable black spots on the display surface or record medium.
Noisy black
In telecommunication, the term noisy black has the following meanings:
1. In facsimile or display systems, such as television, a nonuniformity in the black area of the image, i.e., document or picture, caused by the presence of noise in the received signal.
2. A signal or signal level that is supposed to represent a black area on the object, but has a noise content sufficient to cause the creation of noticeable white spots on the display surface or record medium.
References:
1. ^ http://www.rane.com/par-n.html#noise_color
2. ^ FS-1037C
3. ^ http://www.rane.com/par-n.html#noise_color
4. ^ http://www.ears.dmu.ac.uk/rubrique.php3?id_rubrique=93
5. ^ http://oak.cats.ohiou.edu/~cuddingt/pubs/proceedings99.pdf
6. ^ http://www.ptpart.co.uk/show.php?contentid=71
7. ^ http://www.ptpart.co.uk/show.php?contentid=71
8. ^ http://www.ptpart.co.uk/show.php?contentid=71
9. ^ http://www.engr.uky.edu/~dllau/Halftone/HtmlFiles/paper2.html
10. ^ http://www.ptpart.co.uk/show.php?contentid=71
11. ^ http://www.its.bldrdoc.gov/fs-1037/dir-005/_0649.htm
12. ^ http://www.ptpart.co.uk/show.php?contentid=71
13. ^ http://www.ptpart.co.uk/show.php?contentid=71
* Mitchell, Don P., "Generating Antialiased Images at Low Sampling Densities." Computer Graphics, volume 21, number 4, July 1987.
* Yellott, John I. Jr., "Spectral Consequences of Photoreceptor Sampling in the Rhesus Retina." Science, volume 221, pp. 382-385, 1983.
This article contains material from the Federal Standard 1037C, which, as a work of the United States Government, is in the public domain.
-en.wikipedia.org/wiki/Colors_of_noise
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