title: Audio equalization resolution
body: <h2>In short</h2>
  
  <p>I am trying to make an equalizer in C++. I must process the audio in blocks of (usually) 256 samples. I use an FFT to obtain the spectrum of the sample block. I process the frequencies and then use an IFFT to obtain the processed audio block in the time-domain. I am using the code provided in the related question: <a href="https://stackoverflow.com/questions/8801158/fft-in-a-single-c-file">FFT in a single C-file</a>.</p>
  
  <p>For a low-pass filter, setting the lower coefficients to zero should remove the low frequencies. However, in the processed audio, I get a changed spectrum, rather than a low-pass filter (the audio contains a buzzing sound).</p>
  
  <p>I have read the following related articles:</p>
  
  <ul>
  <li><a href="https://stackoverflow.com/questions/35530/what-are-high-pass-and-low-pass-filters">What Are High-Pass and Low-Pass Filters?</a></li>
  <li><a href="https://stackoverflow.com/questions/11284813/primary-fft-coefficients-vs-low-pass-filter">Primary FFT Coefficients vs Low-Pass Filter</a></li>
  <li><a href="https://stackoverflow.com/questions/14269144/how-to-implement-an-equalizer">How to implement an Equalizer</a></li>
  </ul>
  
  <p>Am I doing something incorrectly? Is the block size too small to obtain a reasonable frequency resolution?</p>
  
  <p>I am familiar with the mathematics of the Fourier transform. However, I do not know the details of the FFT and IFFT transforms. It is possible that the provided code is not implemented correctly. However, when the FFT is done and then an IFFT on the result of the FFT, the original audio block is correctly reconstructed.</p>
  
  <h2>Details</h2>
  
  <p>I am writing a VST and am doing all the processing in the <code>processReplacing()</code> method. This is why I am constrained by the block size.</p>
  
  <p>My idea is to keep last n blocks in memory and calculate the FFT on the last n blocks to obtain a higher frequency resolution, then do the processing on these frequencies, and after the IFFT, replace only the last block.</p>
  
  <p>Are there any suggestions on whether this is a correct way of implementing an equalizer?</p>