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Abstract

A filtering method and apparatus in which an input signal is subjected to an analog filtering step, then an analog to digital conversion step and finally a digital filtering step to produce a filtered output signal. The pulse response of the digital filter is the mirror image of the pulse response of the analog filter and the cascaded filtering steps result in a filtered output signal which exhibits little or no group delay or phase distortion. The digital filter may be a FIR filter. Also disclosed is a phase error correction method whereby a pulse is inserted into a circuit or data stream prior to a group delay imparting device or function, a pulse response of the group delay imparting device or function is measured prior to a phase correcting device or function, mirror image filter coefficients are calculated using a window function and then incorporated into a phase correcting filter arrangement.

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