title: FFT on iPhone to ignore background noise and find lower pitches
body: <p>I have implemented Demetri's <a href="https://github.com/irtemed88/PitchDetector" rel="noreferrer">Pitch Detector</a> project for the iPhone and hitting up against two problems. 1) any sort of background noise sends the frequency reading bananas and 2) lower frequency sounds aren't being pitched correctly. I tried to tune my guitar and while the higher strings worked - the tuner could not correctly discern the low E.</p>
  
  <p>The Pitch Detection code is located in RIOInterface.mm and goes something like this ...</p>
  
  <pre><code>// get the data
  AudioUnitRender(...);
  
  // convert int16 to float
  Convert(...);
  
  // divide the signal into even-odd configuration
  vDSP_ctoz((COMPLEX*)outputBuffer, 2, &amp;A, 1, nOver2);
  
  // apply the fft
  vDSP_fft_zrip(fftSetup, &amp;A, stride, log2n, FFT_FORWARD);
  
  // convert split real form to split vector
  vDSP_ztoc(&amp;A, 1, (COMPLEX *)outputBuffer, 2, nOver2);
  </code></pre>
  
  <p>Demetri then goes on to determine the 'dominant' frequency as follows:</p>
  
  <pre><code>float dominantFrequency = 0;
  int bin = -1;
  for (int i=0; i&lt;n; i+=2) {
      float curFreq = MagnitudeSquared(outputBuffer[i], outputBuffer[i+1]);
      if (curFreq &gt; dominantFrequency) {
          dominantFrequency = curFreq;
          bin = (i+1)/2;
      }
  }
  memset(outputBuffer, 0, n*sizeof(SInt16));
  
  // Update the UI with our newly acquired frequency value.
  [THIS-&gt;listener frequencyChangedWithValue:bin*(THIS-&gt;sampleRate/bufferCapacity)];
  </code></pre>
  
  <p>To start with, I believe I need to apply a LOW PASS FILTER ... but I'm not an FFT expert and not sure exactly where or how to do that against the data returned from the vDSP functions. I'm also not sure how to improve the accuracy of the code in the lower frequencies. There seem to be other algorithms to determine the dominant frequency - but again, looking for a kick in the right direction when using the data returned by Apple's Accelerate framework.</p>
  
  <p>UPDATE:</p>
  
  <p>The accelerate framework actually has some windowing functions. I setup a basic window like this</p>
  
  <pre><code>windowSize = maxFrames;
  transferBuffer = (float*)malloc(sizeof(float)*windowSize);
  window = (float*)malloc(sizeof(float)*windowSize);
  memset(window, 0, sizeof(float)*windowSize);
  vDSP_hann_window(window, windowSize, vDSP_HANN_NORM); 
  </code></pre>
  
  <p>which I then apply by inserting </p>
  
  <pre><code>vDSP_vmul(outputBuffer, 1, window, 1, transferBuffer, 1, windowSize); 
  </code></pre>
  
  <p>before the vDSP_ctoz function. I then change the rest of the code to use 'transferBuffer' instead of outputBuffer ... but so far, haven't noticed any dramatic changes in the final pitch guess.</p>