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Noise reduction is the process of removing noise from a signal. Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree.
Noise rejection is the ability of a circuit to isolate an undesired signal component from the desired signal component, as with common-mode rejection ratio.
All signal processing devices, both analog and digital, have traits that make them susceptible to noise. Noise can be random 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 that occurs due to temperature. These agitated electrons rapidly add and subtract from the voltage of the output signal and thus create detectable noise.