ノッチフィルター（双二次） / Biquad Notch Filter

function [B,A] = biquad_notch(fc, bw, fs)
%BIQUAD_NOTCH   Biquad notch filter
%   [B,A] = biquad_notch(fc, bw, fs)
%
%   Input:
%      fc - cutoff frequency (Hz)
%      bw - bandwidth in octaves (between -3dB frequencies)
%      fs - sampling frequency (Hz)
%
%   2010-01-18 by MARUI Atsushi
%              based on "Cookbook formulae for audio EQ biquad filter
%              coefficients" by Robert Bristow-Johnson.

omega = 2 * pi * fc / fs;
alpha = sin(omega) * sinh(log(2) / 2 * bw * omega / sin(omega));

B = [
   1
  -2 * cos(omega)
   1
]';

A = [
   1 + alpha
  -2 * cos(omega)
   1 - alpha
]';