Hello, and firstly appologies, I am a novice matlab user.

I want to filter some frequencies from a time domain signal (filter out everything below say 7 Hz).  I was expecting to:

FFT time signal to frequency domain

Apply filter to frequency domain signal

inverse FFT filtered signal back to time domain.

What is wrong with this approach? When I plot the results, it seems to have applied the filter to the time domain signal....

Many thanks,  Tom

my .m file so far:

function Hd = tjfilter

Fs = 200; % Sampling frequency
T = 1/Fs; % Sample time
L = 16384; % Length of signal(no. of data lines)
t = (0:L-1)*T; % Time vector

a = load('response_to_z.txt');

t = a(:,1);
x = a(:,2);

figure(1)
plot(t,x)

X = fft(x);

N     = 200;  % Order
Fstop = 7;    % Stopband Frequency
Fpass = 10;    % Passband Frequency
Wstop = 1;    % Stopband Weight
Wpass = 1;  % Passband Weight
dens  = 20;   % Density Factor

% Calculate the coefficients using the FIRPM function.
b  = firpm(N, [0 Fstop Fpass Fs/2]/(Fs/2), [0 0 1 1], [Wstop Wpass], ...
           {dens});
Hd = dfilt.dffir(b);

Y=filter(Hd,X);
fv = [0:0.005:100];

figure(2)
plot(fv,X);
figure(3)
plot(fv,Y);

y = (ifft(Y));
figure(4)
plot(t,y)