function [B,A] = adsgn(Fs);
% ADSGN  Design of a A-weighting filter.
%    [B,A] = ADSGN(Fs) designs a digital A-weighting filter for
%    sampling frequency Fs. Usage: Y = FILTER(B,A,X).
%    Warning: Fs should normally be higher than 20 kHz. For example,
%    Fs = 48000 yields a class 1-compliant filter.
%
%    Requires the Signal Processing Toolbox.
%
%    See also ASPEC, CDSGN, CSPEC.

% Author: Christophe Couvreur, Faculte Polytechnique de Mons (Belgium)
%         couvreur@thor.fpms.ac.be
% Last modification: Aug. 20, 1997, 10:00am.

% References:
%    [1] IEC/CD 1672: Electroacoustics-Sound Level Meters, Nov. 1996.

pkg load signal

% Definition of analog A-weighting filter according to IEC/CD 1672.
f1 = 20.598997;
f2 = 107.65265;
f3 = 737.86223;
f4 = 12194.217;
A1000 = 1.9997;
pi = 3.14159265358979;
NUMs = [ (2*pi*f4)^2*(10^(A1000/20)) 0 0 0 0 ];
DENs = conv([1 +4*pi*f4 (2*pi*f4)^2],[1 +4*pi*f1 (2*pi*f1)^2]);
DENs = conv(conv(DENs,[1 2*pi*f3]),[1 2*pi*f2]);

% Use the bilinear transformation to get the digital filter.
[B,A] = bilinear(NUMs,DENs,1/Fs);