function [B,A] = octdsgn(Fc,Fs,N); % OCTDSGN  Design of an octave filter.%    [B,A] = OCTDSGN(Fc,Fs,N) designs a digital octave filter with %    center frequency Fc for sampling frequency Fs. %    The filter are designed according to the Order-N specification %    of the ANSI S1.1-1986 standard. Default value for N is 3. %    Warning: for meaningful design results, center values used%    should preferably be in range Fs/200 < Fc < Fs/5.%    Usage of the filter: Y = FILTER(B,A,X). %%    Requires the Signal Processing Toolbox. %%    See also OCTSPEC, OCT3DSGN, OCT3SPEC.% Author: Christophe Couvreur, Faculte Polytechnique de Mons (Belgium)%         couvreur@thor.fpms.ac.be% Last modification: Aug. 22, 1997, 9:00pm.% References: %    [1] ANSI S1.1-1986 (ASA 65-1986): Specifications for%        Octave-Band and Fractional-Octave-Band Analog and%        Digital Filters, 1993.if (nargin > 3) | (nargin < 2)  error('Invalide number of arguments.');endif (nargin == 2)  N = 3; endif (Fc > 0.70*(Fs/2))  error('Design not possible. Check frequencies.');end% Design Butterworth 2Nth-order octave filter % Note: BUTTER is based on a bilinear transformation, as suggested in [1]. %W1 = Fc/(Fs/2)*sqrt(1/2);%W2 = Fc/(Fs/2)*sqrt(2); pi = 3.14159265358979;beta = pi/2/N/sin(pi/2/N); alpha = (1+sqrt(1+8*beta^2))/4/beta;W1 = Fc/(Fs/2)*sqrt(1/2)/alpha; W2 = Fc/(Fs/2)*sqrt(2)*alpha;[B,A] = butter(N,[W1,W2]);