1 ## Copyright (C) 2000 Paul Kienzle <pkienzle@users.sf.net>
3 ## This program is free software; you can redistribute it and/or modify it under
4 ## the terms of the GNU General Public License as published by the Free Software
5 ## Foundation; either version 3 of the License, or (at your option) any later
8 ## This program is distributed in the hope that it will be useful, but WITHOUT
9 ## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
10 ## FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
13 ## You should have received a copy of the GNU General Public License along with
14 ## this program; if not, see <http://www.gnu.org/licenses/>.
16 ## usage: [n, Wn, beta, ftype] = kaiserord(f, m, dev [, fs])
18 ## Returns the parameters needed for fir1 to produce a filter of the
19 ## desired specification from a kaiser window:
20 ## n: order of the filter (length of filter minus 1)
21 ## Wn: band edges for use in fir1
22 ## beta: parameter for kaiser window of length n+1
23 ## ftype: choose between pass and stop bands
24 ## b = fir1(n,Wn,kaiser(n+1,beta),ftype,'noscale');
26 ## f: frequency bands, given as pairs, with the first half of the
27 ## first pair assumed to start at 0 and the last half of the last
28 ## pair assumed to end at 1. It is important to separate the
29 ## band edges, since narrow transition regions require large order
31 ## m: magnitude within each band. Should be non-zero for pass band
32 ## and zero for stop band. All passbands must have the same
33 ## magnitude, or you will get the error that pass and stop bands
34 ## must be strictly alternating.
35 ## dev: deviation within each band. Since all bands in the resulting
36 ## filter have the same deviation, only the minimum deviation is
37 ## used. In this version, a single scalar will work just as well.
38 ## fs: sampling rate. Used to convert the frequency specification into
39 ## the [0, 1], where 1 corresponds to the Nyquist frequency, fs/2.
41 ## The Kaiser window parameters n and beta are computed from the
42 ## relation between ripple (A=-20*log10(dev)) and transition width
43 ## (dw in radians) discovered empirically by Kaiser:
45 ## / 0.1102(A-8.7) A > 50
46 ## beta = | 0.5842(A-21)^0.4 + 0.07886(A-21) 21 <= A <= 50
49 ## n = (A-8)/(2.285 dw)
52 ## [n, w, beta, ftype] = kaiserord([1000,1200], [1,0], [0.05,0.05], 11025);
53 ## freqz(fir1(n,w,kaiser(n+1,beta),ftype,'noscale'),1,[],11025);
55 ## TODO: order is underestimated for the final test case: 2 stop bands.
56 ## TODO: octave> ftest("kaiserord") # shows test cases
58 function [n, w, beta, ftype] = kaiserord(f, m, dev, fs)
60 if (nargin<2 || nargin>4)
64 ## default sampling rate parameter
65 if nargin<4, fs=2; endif
68 if length(f)!=2*length(m)-2
69 error("kaiserord must have one magnitude for each frequency band");
71 if any(m(1:length(m)-2)!=m(3:length(m)))
72 error("kaiserord pass and stop bands must be strictly alternating");
74 if length(dev)!=length(m) && length(dev)!=1
75 error("kaiserord must have one deviation for each frequency band");
78 if dev <= 0, error("kaiserord must have dev>0"); endif
80 ## use midpoints of the transition region for band edges
81 w = (f(1:2:length(f))+f(2:2:length(f)))/fs;
85 if m(1)>m(2), ftype='low'; else ftype='high'; endif
87 if m(1)>m(2), ftype='stop'; else ftype='pass'; endif
89 if m(1)>m(2), ftype='DC-1'; else ftype='DC-0'; endif
92 ## compute beta from dev
95 beta = 0.1102*(A-8.7);
97 beta = 0.5842*(A-21)^0.4 + 0.07886*(A-21);
102 ## compute n from beta and dev
103 dw = 2*pi*min(f(2:2:length(f))-f(1:2:length(f)))/fs;
104 n = max(1,ceil((A-8)/(2.285*dw)));
106 ## if last band is high, make sure the order of the filter is even.
107 if ((m(1)>m(2)) == (rem(length(w),2)==0)) && rem(n,2)==1, n = n+1; endif
114 %! subplot(221); bands=[1200, 1500]; mag=[1, 0]; dev=[0.1, 0.1];
116 %! subplot(222); bands=[1000, 1500]; mag=[0, 1]; dev=[0.1, 0.1];
118 %! subplot(223); bands=[1000, 1200, 3000, 3500]; mag=[0, 1, 0]; dev=0.1;
120 %! subplot(224); bands=100*[10, 13, 15, 20, 30, 33, 35, 40];
121 %! mag=[1, 0, 1, 0, 1]; dev=0.05;
123 %! [n, w, beta, ftype] = kaiserord(bands, mag, dev, Fs);
124 %! d=max(1,fix(n/10));
125 %! if mag(length(mag))==1 && rem(d,2)==1, d=d+1; endif
126 %! [h, f] = freqz(fir1(n,w,ftype,kaiser(n+1,beta),'noscale'),1,[],Fs);
127 %! hm = freqz(fir1(n-d,w,ftype,kaiser(n-d+1,beta),'noscale'),1,[],Fs);
128 %! plot(f,abs(hm),sprintf("r;order %d;",n-d), ...
129 %! f,abs(h), sprintf("b;order %d;",n));
130 %! b = [0, bands, Fs/2]; hold on;
131 %! for i=2:2:length(b),
132 %! hi=mag(i/2)+dev(1); lo=max(mag(i/2)-dev(1),0);
133 %! plot([b(i-1), b(i), b(i), b(i-1), b(i-1)],[hi, hi, lo, lo, hi],"c;;");
137 %! %--------------------------------------------------------------
138 %! % A filter meets the specifications if its frequency response
139 %! % passes through the ends of the criteria boxes, and fails if
140 %! % it passes through the top or the bottom. The criteria are
141 %! % met precisely if the frequency response only passes through
142 %! % the corners of the boxes. The blue line is the filter order
143 %! % returned by kaiserord, and the red line is some lower filter
144 %! % order. Confirm that the blue filter meets the criteria and
145 %! % the red line fails.
147 %!# XXX FIXME XXX extend demo to show detail at criteria box corners