Patent ID: 8040944

Claim:
An adaptive digital filter comprising: a filter unit for generating an output signal by means of a convolution operation of an input signal and filter coefficients and supplying the result as output signal; and a coefficient control unit for controlling said filter coefficients based on error between a target signal and an index value derived from said output signal, and further, for controlling said target signal based on at least one of said input signal and said output signal to suppress the range of fluctuation of said filter coefficients; wherein said filter unit takes the input signal that is a complex signal in which one of two signals that are generated from one real signal and that have phases shifted 90° from each other is a real part and the other signal is an imaginary part, generating the output signal that is a complex signal by means of a convolution operation of the input signal and filter coefficients that are real signals, and supplying the result as output signal, and wherein said coefficient control unit comprises: an envelope target value generation circuit for controlling said target signal based on an amplitude of said output signal, decreases said target signal when the amplitude of said output signal is increasing, increases said target signal when the amplitude of said output signal is decreasing, and moreover, makes the speed of decreasing said target signal greater than the speed of increasing said target signal.