Noise reduction is the process of removing noise from a signal.
All signal processing devices, both analog and digital, have traits that make them susceptible to noise. Noise can be random or white noise with an even frequency distribution, or frequency-dependent noise introduced by a device's mechanism or signal processing algorithms.
In electronic recording devices, a major type of noise is hiss created by random electron motion due to thermal agitation at all temperatures above absolute zero. These agitated electrons rapidly add and subtract from the voltage of the output signal and thus create detectable noise.
In the case of photographic film and magnetic tape, noise (both visible and audible) is introduced due to the grain structure of the medium. In the photographic film, the size of the grains in the film determines the film's sensitivity, more sensitive film having larger sized grains. In the magnetic tape, the larger the grains of the magnetic particles (usually a ferric oxide or magnetite), the more prone the medium is to noise.
To compensate for this, larger areas of film or magnetic tape may be used to lower the noise to an acceptable level.
Many noise reduction algorithms tend to alter signals to a greater or lesser degree. The local signal-and-noise orthogonalization algorithm can be used to avoid changes to the signals.