%% Inverse-Dirichlet-Sinc-Shaped Passband
% Design two order 30 constrained equiripple FIR filters with inverse-Dirichlet-sinc-shaped passbands. The cutoff frequency in both designs is pi/4 radians/sample. Set C=1 in one design C=2 in the second design. The maximum passband and stopband ripple is 0.05. Set p=1 in one design and p=2 in the second design.
% Design the filters.

b1 = firceqrip(30,0.25,[0.05 0.05],'invdiric',[1 1]);
b2 = firceqrip(30,0.25,[0.05 0.05],'invdiric',[2 2]);

%
% Obtain the filter frequency responses using freqz. Plot the magnitude responses.

 [h1,~] = freqz(b1,1);
 [h2,w] = freqz(b2,1);
 plot(w,abs(h1)); hold on;
 plot(w,abs(h2),'r');
 axis([0 pi 0 1.5]);
 xlabel('Radians/sample');
 ylabel('Magnitude');
 legend('C=1 p=1','C=2 p=2');

%
% Inspect the stopband ripple in the design with C=1 and p=1. The constrained design sets the maximum ripple to be 0.05. Zoom in on the stopband from the cutoff frequency of pi/4 radians/sample to 3pi/4 radians/sample.

 figure;
 plot(w,abs(h1));
 set(gca,'xlim',[pi/4 3*pi/4]);
 grid on;

% Copyright 2012 The MathWorks, Inc.