NOISE

In common use, the word noise means unwanted sound or noise pollution . In electronics noise can refer to the electronic signal corresponding to acoustic noise (in an audio system) or the electronic signal corresponding to the (visual) noise commonly seen as 'snow' on a degraded television or video image. In signal processing or computing it can be considered data without meaning; that is, data that is not being used to transmit a signal, but is simply produced as an unwanted by-product of other activities. In Information Theory, however, noise is still considered to be information. In a broader sense, film grain or even advertisements in web pages can be considered noise.

Noise can block, distort, or change/interfere with the meaning of a message in both human and electronic communication.In many of these areas, the special case of thermal noise arises, which sets a fundamental lower limit to what can be measured or signaled and is related to basic physical processes at the molecular level described by well-established thermodynamics considerations, some of which are expressible by relatively well known simple formulae. When speaking of noise in relation to sound, what is commonly meant is meaningless sound of greater than usual volume. Thus, a loud activity may be referred to as noisy. However, conversations of other people may be called noise for people not involved in any of them, and noise can be any unwanted sound such as the noise of dogs barking, neighbours playing loud music, road traffic sounds, chainsaws, or aircraft, spoiling the quiet of the countryside.

Electronic noise exists in all circuits and devices as a result of thermal noise, also referred to as Johnson Noise. Semiconductor devices can also contribute flicker noise and generation-recombination noise. In any electronic circuit, there exist random variations in current or voltage caused by the random movement of the electrons carrying the current as they are jolted around by thermal energy. Lower temperature results in lower thermal noise. This same phenomenon limits the minimum signal level that any radio receiver can usefully respond to, because there will always be a small but significant amount of thermal noise arising in its input circuits. This is why radio telescopes, which search for very low levels of signal from stars, use front-end low-noise amplifier circuits, usually mounted on the aerial dish, and cooled with liquid nitrogen.

Noise is also used in the creation of 2d and 3d images by computer. Sometimes noise is added to images to hide the sudden transitions inherent in digital representation of color, known as "banding." This adding of noise is referred to as "dithering." Sometimes noise is used to create the subject matter itself. Procedural noise (such as Perlin noise) is often used to create natural-looking variation in computer generated images.