1 ## Copyright (c) 2007 R.G.H. Eschauzier <reschauzier@yahoo.com>
2 ## Copyright (c) 2011 Carnë Draug <carandraug+dev@gmail.com>
3 ## Copyright (c) 2011 Juan Pablo Carbajal <carbajal@ifi.uzh.ch>
5 ## This program is free software; you can redistribute it and/or modify it under
6 ## the terms of the GNU General Public License as published by the Free Software
7 ## Foundation; either version 3 of the License, or (at your option) any later
10 ## This program is distributed in the hope that it will be useful, but WITHOUT
11 ## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 ## FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
15 ## You should have received a copy of the GNU General Public License along with
16 ## this program; if not, see <http://www.gnu.org/licenses/>.
19 ## @deftypefn{Function File} {[@var{b_out}, @var{a_out}] =} impinvar (@var{b}, @var{a}, @var{fs}, @var{tol})
20 ## @deftypefnx{Function File} {[@var{b_out}, @var{a_out}] =} impinvar (@var{b}, @var{a}, @var{fs})
21 ## @deftypefnx{Function File} {[@var{b_out}, @var{a_out}] =} impinvar (@var{b}, @var{a})
22 ## Converts analog filter with coefficients @var{b} and @var{a} to digital,
23 ## conserving impulse response.
25 ## If @var{fs} is not specificied, or is an empty vector, it defaults to 1Hz.
27 ## If @var{tol} is not specified, it defaults to 0.0001 (0.1%)
28 ## This function does the inverse of impinvar so that the following example should
29 ## restore the original values of @var{a} and @var{b}.
31 ## @command{invimpinvar} implements the reverse of this function.
33 ## [b, a] = impinvar (b, a);
34 ## [b, a] = invimpinvar (b, a);
37 ## Reference: Thomas J. Cavicchi (1996) ``Impulse invariance and multiple-order
38 ## poles''. IEEE transactions on signal processing, Vol 40 (9): 2344--2347
40 ## @seealso{bilinear, invimpinvar}
43 function [b_out, a_out] = impinvar (b_in, a_in, fs = 1, tol = 0.0001)
49 ## to be compatible with the matlab implementation where an empty vector can
50 ## be used to get the default
54 ts = 1/fs; # we should be using sampling frequencies to be compatible with Matlab
57 [r_in, p_in, k_in] = residue(b_in, a_in); % partial fraction expansion
59 n = length(r_in); % Number of poles/residues
61 if (length(k_in)>0) % Greater than zero means we cannot do impulse invariance
62 error("Order numerator >= order denominator");
65 r_out = zeros(1,n); % Residues of H(z)
66 p_out = zeros(1,n); % Poles of H(z)
67 k_out = 0; % Contstant term of H(z)
72 first_pole = p_in(i); % Pole in the s-domain
73 while (i<n && abs(first_pole-p_in(i+1))<tol) % Multiple poles at p(i)
75 m++; % Next multiplicity
77 [r, p, k] = z_res(r_in(i-m+1:i), first_pole, ts); % Find z-domain residues
78 k_out += k; % Add direct term to output
79 p_out(i-m+1:i) = p; % Copy z-domain pole(s) to output
80 r_out(i-m+1:i) = r; % Copy z-domain residue(s) to output
82 i++; % Next s-domain residue/pole
85 [b_out, a_out] = inv_residue(r_out, p_out, k_out, tol);
86 a_out = to_real(a_out); % Get rid of spurious imaginary part
87 b_out = to_real(b_out);
89 % Shift results right to account for calculating in z instead of z^-1
94 ## Convert residue vector for single and multiple poles in s-domain (located at sm) to
95 ## residue vector in z-domain. The variable k is the direct term of the result.
96 function [r_out, p_out, k_out] = z_res (r_in, sm, ts)
98 p_out = exp(ts * sm); % z-domain pole
99 n = length(r_in); % Multiplicity of the pole
100 r_out = zeros(1,n); % Residue vector
102 %% First pole (no multiplicity)
103 k_out = r_in(1) * ts; % PFE of z/(z-p) = p/(z-p)+1; direct part
104 r_out(1) = r_in(1) * ts * p_out; % pole part of PFE
106 for i=(2:n) % Go through s-domain residues for multiple pole
107 r_out(1:i) += r_in(i) * polyrev(h1_z_deriv(i-1, p_out, ts)); % Add z-domain residues
113 %!function err = stozerr(bs,as,fs)
115 %! % number of time steps
118 %! % impulse invariant transform to z-domain
119 %! [bz az]=impinvar(bs,as,fs);
121 %! % create sys object of transfer function
124 %! % calculate impulse response of continuous time system
125 %! % at discrete time intervals 1/fs
126 %! ys=impulse(s,(n-1)/fs,1/fs)';
128 %! % impulse response of discrete time system
129 %! yz=filter(bz,az,[1 zeros(1,n-1)]);
132 %! err=sqrt(sum((yz*fs.-ys).^2)/length(ys));
135 %!assert(stozerr([1],[1 1],100),0,0.0001);
136 %!assert(stozerr([1],[1 2 1],100),0,0.0001);
137 %!assert(stozerr([1 1],[1 2 1],100),0,0.0001);
138 %!assert(stozerr([1],[1 3 3 1],100),0,0.0001);
139 %!assert(stozerr([1 1],[1 3 3 1],100),0,0.0001);
140 %!assert(stozerr([1 1 1],[1 3 3 1],100),0,0.0001);
141 %!assert(stozerr([1],[1 0 1],100),0,0.0001);
142 %!assert(stozerr([1 1],[1 0 1],100),0,0.0001);
143 %!assert(stozerr([1],[1 0 2 0 1],100),0,0.0001);
144 %!assert(stozerr([1 1],[1 0 2 0 1],100),0,0.0001);
145 %!assert(stozerr([1 1 1],[1 0 2 0 1],100),0,0.0001);
146 %!assert(stozerr([1 1 1 1],[1 0 2 0 1],100),0,0.0001);