Question

A filter with a positive phase shift is non causal, it looks into the future . this is not possible .what is really happening ??? ?

Answer 1

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Technically, you are right, a filter with a positive phase shift looks into the future. However, note that mathematically, any positive phase shift can be represented as a negative phase shift also, because of the property of the phase folding around 360⁰ . For example, sin(t+270⁰) can be rewritten as sin(t+360⁰ - 30⁰) = sin(t - 30⁰). Thus, all "look ahead" non causal filters can be represented by a causal counterpart !!