NONCAUSAL COMPLEMENTARY FILTERS

The design of complementary IIR lowpass and highpass filters is transferred into the sphere of non-causal filtering. Thus, very good insight into the properties of these filters is achieved. Then, starting with the general form of the transfer function, a new Remez-type design procedure is discussed. Its characteristic feature is that for a given filter order, the number of poles outside the unit circle can be selected. From the resulting wide class of filters, further possibilities are distinguished, like the maximum-ripple case and filters with fixed passband or stopband edge frequency. Along with a well-known method for realizing anticausal sections, to which two improvements are added, possible applications to both causal and non-causal IIR filtering are demonstrated.