Noise Defined
Douglas Self

   By Guest Contributor   Categories: General

Noise here refers only to the random noise generated by resistances and active devices. The term is sometimes used to include mains hum, spurious signals from demodulated RF and other non-random sources, but this threatens confusion and I prefer to call the other unwanted signals ‘interference’. In one case we strive to minimize the random variations arising in the circuit itself, in the other we are trying to keep extraneous signals out, and the techniques are wholly different.

When noise is referred to in electronics it means white noise unless it is specifically labeled as something else, because that is the form of noise that most electronic processes generate. There are two elemental noise mechanisms that make themselves felt in all circuits and active devices. These are Johnson noise and shot noise, which are both are forms of white noise. Both have Gaussian probability density functions. These two basic mechanisms generate the noise in both bipolar junction transistors (BJTs) and field-effect transistors (FETs), though in rather different ways.

There are other forms of noise that originate from less fundamental mechanisms such as device processing imperfections that do not have a white spectrum; examples are 1/f (flicker) noise and popcorn noise. These noise mechanisms are described later in this chapter.

Non-white noise is given a color that corresponds to the visible spectrum; thus, red noise has a larger low-frequency content than white noise, while pink is midway between the two.

White noise has equal power in equal absolute bandwidth, i.e. with the bandwidth measured in Hz. Thus, there is the same power between 100 and 200 Hz as there is between 1100 and 1200 Hz. It is the type produced by most electronic noise mechanisms.

Pink noise has equal power in equal ratios of bandwidth, so there is the same power between 100 and 200 Hz as there is between 200 and 400 Hz. The energy per Hz falls at 3 dB per octave as frequency increases. Pink noise is widely used for acoustic applications like room equalization and loudspeaker measurement as it gives a flat response when viewed on a third-octave or other constant-percentage-bandwidth spectrum analyzer.

Red noise has energy per Hz falling at 6 dB per octave rather than 3. It is important in the study of stochastic processes and climate models, but has little application in audio. The only place you are likely to encounter it is in the oscillator section of analog synthesizers. It is sometimes called Brownian noise as it can be produced by Brownian motion; hence its alternative name of random-walk noise. Brown here is a person and not a color.

Blue noise has energy per Hz rising at 3 dB per octave. Blue noise is used for dithering in image anti-aliasing, but has, as far as I am aware, no application to audio. The spectral density of blue noise (i.e. the power per Hz) is proportional to the frequency. It appears that the lightsensitive cells in the retina of the mammalian eye are arranged in a pattern that resembles blue noise [4]. Great stuff, this evolution.

Violet noise has energy per Hz rising at 6 dB per octave (I imagine you saw that one coming). It is also known as ‘differentiated white noise’ as a differentiator circuit has a frequency response rising at 6 dB per octave. It is sometimes called purple noise.

Gray noise is pink noise modified by a psychoacoustic equal loudness curve, such as the inverse of the A-weighting curve, to give the perception of equal loudness at all frequencies.

Green noise really does exist, though not in the audio domain. It is used for stochastic half-toning of images, and consists of binary dither patterns composed of homogeneously distributed minority pixel clusters. I think we had better leave it there.

About the Author

Douglas wears his learning lightly, and this book features the engaging prose style familiar to readers of his other books The Audio Power Amplifier Design Handbook and Self on Audio. You will learn why mercury cables are not a good idea, the pitfalls of plating gold on copper, and what quotes from Star Trek have to do with PCB design.

Excerpt from Douglas Self’s Small Signal Audio Design .

 

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