ADAPTIVE DIGITAL FILTER

摘要

TITLE OF THE INVENTION An Adaptive Digital Filter An adaptive digital filter has been found which can be used, in a prediction filter residing in an ADPCM modulator and an ADPCM demodulator, providing stable operation and minimum phase shift response. The filter (Fig.4, Fig.5) has three parallel branches (A,B,C), two (A,B) of them have a plurality of series connected non-recursive filter elements (A1-An, B1-Bn) each of which has a degree not larger than three relating to an operator (Z-1) with the third branch (C) being merely a conductive line. The tap coefficients (c1, c2, ....,d1, d2,...) of non-recursive filter elements change according to solutions of the transfer function of the filter. A numerator and/or a denominator of the transfer function of the present filter is a Chebychev polynominal relating to an operator (Z-1), and has a zero point and/or a pole. The solutions (wi, vi) of a numerator and a denominator are located alternatively on a unit circle (Fig. 6) in the Z-1 plane. When the solutions of the transfer function do not locate alternatively on said unit circle, tap coefficients (c1, c2, ...., d1, d2....) are not updated since non-alternate solutions do not assure the stable operation and/or minimum phase shift response of the filter.