function [Bp,Ap] = boost3a(fs, fc, bw, gain, P);
%BOOST - Design a boost filter at given gain, center
%        frequency fc, bandwidth bw, and sampling rate fs
%        (default = 1).
%
%        Cookbook formulae for audio EQ biquad filter coefficients
%        by Robert Bristow-Johnson  <rbj@audioimagination.com>

Q = bw/fs;

Bp = zeros(P, 3);
Ap = zeros(P, 3);
for p=1:P,
	kp = IIRCalcQp(p, P);
	[Bp(p,:),Ap(p,:)] = peaking(fs, fc, kp*Q, gain);
endfor

B = Bp(1,:);
A = Ap(1,:);
if nargout==0
  close
  freqz(B,A);
  title('Boost Frequency Response')
end

endfunction

function [B,A] = peaking(fs, fc, Q, K)
	k1=-cos(2*pi*fc/fs);
	k2=(1-tan(Q/2))/(1+tan(Q/2));
	Pz = [1 k1*(1+k2) k2]; % define denominator coefficients
	Qz = [k2 k1*(1+k2) 1]; % define numerator coefficients
	Num = (Pz*(1+K) + Qz*(1-K))/2;
	Den = Pz;
	B = Num;
	A = Den;
	
endfunction

function kp = IIRCalcQp(p, P)
	kp = 1.0/(2*sin(pi*(2*p-1)/(2*P)));
	
endfunction