Data that obscures or corrupts signal, as that term is used in the expression signal-to-noise ratio. Although noise is generally unwanted and signal is wanted, there are exceptions. In some circumstances, for example, dithering, which produces noise, is deliberately employed to counteract the aliasing that results when certain frequencies in a sound or image interact with the sampling frequencies applied by digital-capture systems. While noise is often thought of as a random phenomenon, it may be either random or systematic (patterned).