ピーク・フィルター（双二次） / Biquad Peaking Filter

function [B,A] = biquad_peak(fc, gain, q, fs)
%BIQUAD_PEAK   Biquad peak/dip filter
%   [B,A] = biquad_peak(fc, gain, q, fs)
%
%   Input:
%      fc - cutoff frequency (Hz)
%      gain - gain at the peak (dB)
%      q - Q factor
%      fs - sampling frequency (Hz)
%
%   2005-10-13 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) / q;
a = 10 ^ (gain/40);

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

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