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// Copyright (C) 2018 - IIT Bombay - FOSSEE
//
// This file must be used under the terms of the CeCILL.
// This source file is licensed as described in the file COPYING, which
// you should have received as part of this distribution. The terms
// are also available at
// http://www.cecill.info/licences/Licence_CeCILL_V2-en.txt
// Original Source : https://octave.sourceforge.io/signal/
// Modifieded by:Sonu Sharma, RGIT Mumbai
// Organization: FOSSEE, IIT Bombay
// Email: toolbox@scilab.in
function [Zz, Zp, Zg] = bilinear(Sz, Sp, Sg, T)
//Transforms a s-plane filter (Analog) into a z-plane filter (Digital) using Bilinear transformation
//Calling Sequence
// [Zb, Za] = bilinear(Sb, Sa, T)
// [Zb, Zb] = bilinear(Sz, Sp, Sg, T)
// [Zz, Zp, Zg] = bilinear(...)
//Prameters
//Sb: Numerator coefficient vector in s-domain
//Sa: denumerator coefficient vector s-domain
//Sz: zeros in s-plane
//Sp: poles in s-plane
//Sg: gain in s-domain
//T: Sampling period (double)
//Zb: Numerator coefficient vector in z-domain
//Za: denumerator coefficient vector z-domain
//Zz: zeros in z-plane
//Zp: poles in z-plane
//Zg: gain in z-domain
//Description:
//a filter design can be transformed from the s-plane to the z-plane while maintaining the band edges by means of the bilinear transform. This maps the left hand side of the s-plane into the interior of the unit circle in z-plane. The mapping is highly non-linear, so you must design your filter with band edges in the s-plane positioned at 2/T tan(w*T/2) so that they will be positioned at w after the bilinear transform is complete.
//It does following transformation from s-plane to z-plane
// 2 z-1
// s -> - ----
// T z+1
//Examples
//[b a] = bilinear ([1 2 3], [4 5 6], 1, 1)
//Output :
// a =
//
// 1. 7.3333333 17.666667 14.
// b =
//
// 0. - 0.1666667 - 0.3333333 2.5
funcprot(0);
[nargout nargin] = argn();
ieee(2);
if nargin==3
T = Sg;
[Sz, Sp, Sg] = tf2zp(Sz, Sp);
elseif nargin~=4
error("bilinear: invalid number of inputs")
end
p = length(Sp);
z = length(Sz);
if z > p | p==0
error("bilinear: must have at least as many poles as zeros in s-plane");
end
// ---------------- ------------------------- ------------------------
// Bilinear zero: (2+xT)/(2-xT) pole: (2+xT)/(2-xT)
// 2 z-1 pole: -1 zero: -1
// S -> - --- gain: (2-xT)/T gain: (2-xT)/T
// T z+1
// ---------------- ------------------------- ------------------------
Zg = real(Sg * prod((2-Sz*T)/T) / prod((2-Sp*T)/T));
if Zg == 0 & nargout == 3 then
error("bilinear: invalid value of gain due to zero(s) at infinity avoid z-p-g form and use tf form ")
end
Zp = (2+Sp*T)./(2-Sp*T);
SZp = size(Zp);
if isempty(Sz)
Zz = -ones(SZp(1), SZp(2));
else
Zz = [(2+Sz*T)./(2-Sz*T)];
Zz = postpad(Zz, p, -1);
end
if nargout==2
// zero at infinity
Zz1 = [];
for i=1:length(Zz)
if Zz(i) ~= %inf
Zz1 = [Zz1 Zz(i)];
end
end
Zz = Zz1;
if Zg == 0
z = %z;
bi = (2*(z - 1))/(T*(z + 1));
Hs = Sg * real(poly(Sz, "s"))/real(poly(Sp, "s"));
Hz = horner(Hs, bi);
b = coeff(Hz.num);
a = coeff(Hz.den);
Zg = b($)/a($);
end
[Zz, Zp] = zp2tf(Zz, Zp, Zg);
Zz = prepad(Zz, length(Zp));
end
ieee(0);
endfunction
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