As depicted, copies of formula_27 are shifted by multiples of the sampling rate formula_4 and combined by addition. For a band-limited function  formula_35  and sufficiently large formula_15 it is possible for the copies to remain distinct from each other. But if the Nyquist criterion is not satisfied, adjacent copies overlap, and it is not possible in general to discern an unambiguous formula_37 Any frequency component above formula_12 is indistinguishable from a lower-frequency component, called an "alias", associated with one of the copies. In such cases, the customary interpolation techniques produce the alias, rather than the original component. When the sample-rate is pre-determined by other considerations (such as an industry standard), formula_1 is usually filtered to reduce its high frequencies to acceptable levels before it is sampled. The type of filter required is a lowpass filter, and in this application it is called an anti-aliasing filter.