%
%  lect11c.m -- code for FFT -- noise filtering
%
%- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 

%  set grids

L= pi;  N = 256;  h = 2*L/N;

x  = 0:h:2*L-h;
n = -N/2:1:N/2-1;

%  clean & dirty functions

f  = sin(7*pi*x/L);
f1 = f + 0.5*randn(1,N);

%  fft filtering

c = fftshift(fft(f1))/N;
filt = (1-tanh(2*(n-10))).*(1+tanh(2*(n+10)))/4;
f2   = ifft(fftshift(c.*filt)*N);

%  plot

figure(1);clf
subplot(2,1,1)
plot(x,f1)
axis([0 2*L -2 2])
title('\bf noisy wave')
xlabel('\bf x-axis');   ylabel('\bf y-axis'); 

subplot(2,1,2)
plot(x,real(f),'b--')
hold on
plot(x,real(f2),'r')
axis([0 2*L -2 2])
title('\bf original (dashed) vs filtered (solid) waves')
xlabel('\bf x-axis');   ylabel('\bf y-axis'); 

figure(2);clf
subplot(2,1,1)
plot(n,abs(c),'x')
hold on
plot(n,filt)
axis([-N/2 N/2 0 1.1])
title('\bf noisy spectrum & filter profile')
xlabel('\bf n-axis');   ylabel('\bf |c_n|-axis'); 

subplot(2,1,2)
plot(n,abs(c.*filt),'x')
axis([-N/2 N/2 0 1.1])
title('\bf filtered spectrum')
xlabel('\bf n-axis');   ylabel('\bf |c_n|-axis');