低域フィルター（双二次） / Biquad Lowpass Filter

function [B,A] = biquad_lpf(fc, q, fs)
%BIQUAD_LPF   biquad low-pass filter
%    [B,A] = biquad_lpf(fc, q, fs)
%
% Input:
%    fc - cutoff frequency (Hz)
%     q - Q-factor
%    fs - sampling frequency (Hz)
%
% 2005-02-14 by MARUI Atsushi
%            based on "Cookbook formulae for audio EQ biquad filter
%            coefficients" by Robert Bristow-Johnson.
% 2010-01-18 updated to conform better to Bristow-Johnson's definition

omega = 2 * pi * fc / fs;
alpha = sin(omega)/(2*q);

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

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