title: How to filter in the frequency domain
body: <p>I have a signal and I have a filter.</p>
  
  <p>I want to convert the signal into frequency domain and then filter it with my filter.</p>
  
  <p>Converting the signal into frequency domain is easy, but how do I filter the signal now?</p>
  
  <p>This is my filter:</p>
  
  <pre><code>filter_2 = firceqrip(2,0.6,[0.05 0.03]);
  </code></pre>
  
  <p>This is my signal:</p>
  
  <pre><code>freqs = [0.08, 0.2, 0.32, 0.4];
  periods = 1./ freqs;
  t_max = 4 * periods(1);
  t = linspace(0, t_max, 50);
  
  signal = sin(2*pi*0.08*t) + sin(2*pi*0.2*t) + sin(2*pi*0.32*t) + sin(2*pi*0.4*t);
  
  % to find the fft of the signal
  N = 64;
  signal_spect = abs (fft(signal,N));
  signal_spect = fftshift(signal_spect);
  F = [-N/2:N/2-1]/N;
  </code></pre>
  
  <p>now how do I filter the previous signal with my filter?
  I tried doing this fft(filter_2) but it didn't work out, it's probably a wrong approach, but I do not know what else to do</p>
  
  <p>any help is appreicated</p>