libKeyFinder can be used to estimate the musical key of digital recordings.

It is the basis of the KeyFinder GUI app, which is available as a binary download for Mac OSX and Windows at www.ibrahimshaath.co.uk/keyfinder

For the most basic use case, do something like this:

  // Static because it retains useful resources for repeat use
  static KeyFinder::KeyFinder k;

  // Build an empty audio object
  KeyFinder::AudioData a;

  // Prepare the object for your audio stream
  a.setFrameRate(yourAudioStream.framerate);
  a.setChannels(yourAudioStream.channels);
  a.addToSampleCount(yourAudioStream.length);

  // Copy your audio into the object
  while (int i = 0; i < yourAudioStream.length; i++) {
    a.setSample(i, yourAudioStream[i]);
  }

  // Run the analysis
  KeyFinder::KeyDetectionResult r =  k.keyOfAudio(a);

  // And do something with the result!
  doSomethingWith(r.globalKeyEstimate);

Alternatively, you can transform a stream of audio into a chromatic representation, and make progressive estimates of the key:

  KeyFinder::AudioData a;
  a.setFrameRate(yourAudioStream.framerate);
  a.setChannels(yourAudioStream.channels);
  a.addToSampleCount(yourAudioStream.packetLength);

  static KeyFinder::KeyFinder k;
  // the workspace holds the memory allocations for analysis of a single track
  KeyFinder::Workspace w;

  while (yourPacket = newAudioPacket()) {
    while (int i = 0; i < newAudioPacket.length; i++) {
      a.setSample(i, newAudioPacket[i]);
    }
    k.progressiveChromagram(a, w);
    // if you want to grab progressive key estimates...
    KeyFinder::KeyDetectionResult r = k.keyOfChromagram(w);
    doSomethingWithMostRecentKeyEstimate(r.globalKeyEstimate);
  }

  // if you want to squeeze every last bit of audio from the working buffer...
  k.finalChromagram(w)

  // and finally...
  KeyFinder::KeyDetectionResult r = k.keyOfChromagram(w);

  doSomethingWithFinalKeyEstimate(r.globalKeyEstimate);