%====================================================================
%===== The Coefficients of The Filter Designed To The Equation: =====
%=====                   h(n)=(-1)^n h(n)LP                     =====
%====================================================================

Fs=400;
wn=[0.25];
n=(1:1:201);
p=(-1).^n;
[b,a]=fir1(200,wn);
c=b.*p;
[H,w]=freqz(b,a,512);
[h,w]=freqz(c,a,512);

figure(1);
plot(w*Fs/(2*pi),abs(H),w*Fs/(2*pi),abs(h));
title('BAND STOP Filter Frequency Response');
xlabel('Frequency (Hz)');
ylabel('Mangitude of Filter Response');
grid on

%This Is A Band Stop Filter Which Is Extremely 
%Difficult To Construct Using Analogue Techniques.