sos2tf.m

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function [b,a] = sos2tf(sos)
%SOS2TF 2nd-order sections to transfer function linear system model conversion
%   [B,A] = SOS2TF(SOS) returns the numerator and denominator coefficients 
%   B and A of the discrete-time linear system given by SOS in second-order
%   sections form.
%
% 	SOS is an L by 6 matrix which contains the coefficients of each 
%   second-order section in its rows:
%       SOS = [ b01 b11 b21  a01 a11 a21 
%               b02 b12 b22  a02 a12 a22
%               ...
%               b0L b1L b2L  a0L a1L a2L ]
%   The system transfer function is the product of the second-order transfer
%   functions of the sections.  Each row of the SOS matrix describes
%   a 2nd order transfer function as
%       b0k +  b1k z^-1 +  b2k  z^-2
%       ----------------------------
%       a0k +  a1k z^-1 +  a2k  z^-2
%   where k is the row index.
%
%   See also ZP2SOS, SOS2ZP, SOS2SS, SS2SOS

%   Author(s): T. Krauss, 1993
%   Copyright (c) 1988-98 by The MathWorks, Inc.
%   $Revision: 1.7 $  $Date: 1997/11/26 20:13:19 $
 
if nargin ~= 1,
    error('Requires one input argument.')
end

L = size(sos,1);
b = 1;
a = 1;
for i=1:L,
    b1 = sos(i,1:3);
    a1 = sos(i,4:6);
    if b1(3) == 0,     % strip zero coefficients
        if b1(2) == 0,
            b1(2:3) = []; 
        else
            b1(3) = []; 
        end
    end
    if a1(3) == 0,     % strip zero coefficients
        if a1(2) == 0,
            a1(2:3) = []; 
        else
            a1(3) = []; 
        end
    end
    b = conv(b,b1);
    a = conv(a,a1);
end