# Translations:Frequency response of a digital system/70/en

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The concept of minimum phase is attributable to Bode (1945). Let be the poles and let be the zeros of a system. A minimum-phase (or minimum-delay) system is a stable causal system that has its zeros as well as its poles lying outside of the unit circle. That is, stability requires that , whereas minimum delay requires in addition that . The *canonical representation theorem* (Robinson, 1963, theorem 1, p. 203) can be stated as follows: