[sdiy] Yet another analogue synth

Wed May 23 15:01:10 CEST 2001

Happy Harry wrote:
> 
> There was some web site that listed ALL colors of noise...

I lost the link, but here's the text:

    Edu Silva / ES2 Audio -   Salvador / BA / Brazil
    bahi0387 at terra.com.br - http://audiolist.cjb.net

           ------------   The Colors of Noise   ---------------

                             (Martin Saxon)

I thought this Usenet message quite interesting. 

Also, at the end of this page are: 

    a comment by J. Russell Lemon regarding green noise 
    a comment by Michael Golanbari regarding pink noise 
    a comment by David Thall regarding grey (gray) noise 

From: "Joseph S. Wisniewski" <jwisniew at ford.com> 
Newsgroups: comp.dsp, comp.sys.ibm.pc.soundcard.tech, comp.speech,
alt.sci.physics.acoustics

Subject: The colors of noise 1.3
Date: Mon, 07 Oct 1996 08:22:31 -0400
Organization: Ford Motor Company
Lines: 107
Message-ID: <3258F607.ABD at ford.com>
Colors of noise pseudo FAQ, version 1.3 

------------------------------------------------------------------------ 

That email just keeps coming in. So, here's the latest rev. Thanks to
the many people who pointed out the flaws in my pink and blue
definitions. Thanks Kev fot the pointer to FS-1037C. Due to popular
demand, I am reversing my previous stand and adding the definition of
orange noise. 

The noises are now in spectral order (artistic license has been taken
over where white, black, grey, and brown fit into a spectrum). Anyone is
welcome to help fill in the gaps. We're up to three defintions of black
noise. Keep them coming! 

------------------------------------------------------------------------ 

White noise (common definition) power density is constant over a finite
frequency range. AKA Johnson noise. 

Pink noise (common definition) power density decreases 3dB per octave
with increasing frequency (density proportional to 1/f) over a finite
frequency range which does not include DC. Each octave contains the same
amount of power. Many point out that this is not a trivial filtering
problem. AKA flicker noise. 

Red noise (common definition within the oceanographic field, contributed
by P.J. "Josh" Rovero) (Anyone have the spectrum?) 

    oceanic ambient noise (ie, noise distant from the sources) is often
    described as "red" due to the selective absorption of higher
    frequencies." 

Orange noise (anonymous contribution) (Anyone foolish enough to want the
spectrum?) 

    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 system of music 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. Orange noise is most easily generated by a roomfull
    of primary school students equipped with plastic soprano recorders. 

Green noise (defined by some folks producing relaxation tapes, Mystic
Moods, I believe) 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 500Hz. (Anyone have the
spectrum?) 

Blue noise (FS-1037C) power density increases 3dB per octave with
increasing frequency (density proportional to f) over a finite frequency
range. This can be good noise for dithering. 

Purple noise (origional definition, contributed by Jon Risch) power
density increases 6dB per octave with increasing frequency (density
proportional to f^2) over a finite frequency range. Differentiated white
noise. AKA violet noise. 

Grey noise (heard this one a couple of times, but can't put my finger on
a source) noise subjected to a psychoacoustic equal loudness curve (such
as an inverted a-weight curve) over a given range of frequencies, so
that it sounds like it is equally loud at all frequencies. This would be
a better definiton of "white noise" than the "equal power at all
frequencies" definition, since real "white light" has the power spectrum
of a 5400K black body, not an equal power spectrum. 

Brown noise (Jon M. Risch, rbmccammon) power density decreases 6dB per
octave with increasing frequency (density proportional to 1/f^2) over a
frequency range which does not include DC. Is not named for a power
spectrum that suggests the color brown, rather, the name is a coruption
of Brownian motion. If we were going to pick a color, red might be good
since pink noise lies between this noise and white noise. Unfortuantly,
red is already taken. AKA "random walk" or "drunkard's walk" noise. 

**** Three different definitions of black (silent) noise **** 

Black noise (contributed by Jeff Mercure, his own definition) whatever
comes out of an active noise control system and cancles an existing
noise, leaving the world world 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 an annoying tendancy to portray active
noise control in this light.) 

Black noise (seen in the sales literature for an ultrasonic vermin
repeller) power density is constant for a finite frequency range above
20kHz. Ultrasonic white noise. This black noise is like the so-called
"black light" with frequencies too high to be preceived as sound, but
still capable of affecting you or your surroundings. 

Black noise (Manfred Schroeder, "fractals, chaos, power laws,"
contributed by Mike Arnao) 

    has an f ^ -beta spectrum, with beta > 2, and is 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." 

-- 

Joseph S. Wisniewski | Views expressed are my own, and don't reflect
__Ford Motor Company | those of the Ford Motor Co. or affiliates. 
____Project Sapphire | Trans Am, Daytona, Bonneville, and IROC are
___jwisniew at ford.com | just races, won by people driving Ford cars! 

Comments on the Colors of Noise

From: J. Russell Lemon [mailto:Lemon.J.Russell at Worldnet.ATT.net] 

I started using Fortran 4 in 1968 when I was assigned to the Dignital
Communications Experimental Facility at Rome Air Development Center at
Griffiss AFB, N.Y. 

It is there I first saw the U. S. Government spec for "Green Noise"
which was used to simulate voice bandwidth for testing communication
channels. As I remember, it was flat from 500 Hz to about 2 kHz and then
rolled off at 9 db/octave. Below 500 Hz it rolled off at 6 db ?? per
octave. Not sure. It was a long time ago, far from my home in
California. 

From: Michael Golanbari &ltgolanbar at alantro.com> 

You probably are already aware of this, but just in case: Pink noise is
also commonly used to model phase noise in electronic communications
systems. 

From: David Thall &ltretticac at mindspring.com> 

GrayNoise can be defined as a time domain waveform where each next
sample is chosen by shuffling bits in a digital word of 'N'-bits length.
It has also been called 'bit-flip' noise is some obscure literature, and
creates a low rumbly, grainy noise. 

Its spectrum is very flat from about 7 hz - 500 hz, and then falls off
at about 3db / octave up to the nyquist frequency. Of all the digital
noises, it has the highest ratio (or rather, the closest) of peak to RMS
signal energy. 

.