Electronic noise and communication channel noise
Electronic noise exists in all circuits and devices as a result of thermal noise, also referred to as Johnson Noise. It is caused by random variations in current or voltage caused by the random movement of charge carriers (usually electrons) carrying the current as they are jolted around by thermal energy. Thermal noise can be reduced by reducing the temperature of the circuit. This 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 space, use front-end low-noise amplifier circuits cooled with liquid nitrogen.
There are several other sources of noise in electronic circuits such as shot noise, seen in very low-level signals where the finite number of energy-carrying particles becomes significant, or flicker noise (1/f noise) in semiconductor devices. A digitized and reconstructed analog signal is exposed to additive quantization noise.
In a communication channel, noise is an undesired random signal, often modelled as additive white gaussian noise (AWGN), that may be caused by thermal noise orelectromagnetic interference (EMI) from unknown sources. Noise shold not be confused with crosstalk and other interference from other communication system transmitters. Phaseor frequency modulated communication systems may suffer from phase noise due to synchronization problems and time-invariant channel conditions, caused by mobility, fadingand doppler shift. Deliberate generation of communication system noise and interference is called jamming.
Noise is also present in images. Electronic noise will be present in camera sensors, and the physical size of the grains of film emulsion creates visual noise. This kind of noise is referred to as "grain."
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.
The Earth ground and the built environment are subjected to permanent vibrations commonly referred to as seismic noise but nowadays preferably called Ambient Vibrations. These vibrations have natural (ocean waves, wind...) and anthropogenic (traffic, machines...) origins depending on the considered frequency range. These vibrations may be disturbing for people (housing close to railway tracks...) and even cause solidian noise (acoustic noise created by particular vibrations of solids like rooms) but they are also used (recorded) to characterize the structures (civil engineering structure, underground) they are shaking in terms of dynamic properties and eventually physical properties.
The activity and regulation of our genes are also subject to noise. Transcriptional noise refers to the variability in gene activity between cells in genetically identical populations (even identical twins are non-identical). Noise in gene activity has tremendous consequences on cell behaviour, and must be mitigated or integrated. Noise impacts upon the effectiveness of clinical treatment, with resistance of bacteria to antibiotics demonstrably caused by non-genetic differences. Variability in gene expression may also contribute to resistance of sub-populations of cancer cells to chemotherapy. In certain contexts, such as the survival of microbes in rapidly changing stressful environments, or several types of scattered differentiation, noise may be essential.