title: find RMS value in frequency domain
body: <p>i would like to  find RMS value in frequency domain,i used this site for help</p>
  
  <p><a href="http://www.dsprelated.com/showmessage/123177/1.php" rel="nofollow">http://www.dsprelated.com/showmessage/123177/1.php</a></p>
  
  <p>but there it is calculated using FFT,but what if i am using periodgram?instead of this code</p>
  
  <pre><code>X = fft(x)
  
  Parseval's Theorem
  
  sum(x.^2) =  sum(abs(X).^2)/N
  
  RMS == sqrt(sum(x.^2))/N) = sqrt(sum(abs(X/N).^2))
  </code></pre>
  
  <p>how does code  change  in case of periodogram?thanks in advance</p>