feature detection - Page 53

Why is this beat detection code failing to register some beats properly?

- by Quincy

I made this SoundAnalyzer class to detect beats in songs: class SoundAnalyzer { public SoundBuffer soundData; public Sound sound; public List<double> beatMarkers = new List<double>(); public SoundAnalyzer(string path) { soundData = new SoundBuffer(path); sound = new Sound(soundData); } // C = threshold, N = size of history buffer / 1024 B = bands public void PlaceBeatMarkers(float C, int N, int B) { List<double>[] instantEnergyList = new List<double>[B]; GetEnergyList(B, ref instantEnergyList); for (int i = 0; i < B; i++) { PlaceMarkers(instantEnergyList[i], N, C); } beatMarkers.Sort(); } private short[] getRange(int begin, int end, short[] array) { short[] result = new short[end - begin]; for (int i = 0; i < end - begin; i++) { result[i] = array[begin + i]; } return result; } // get a array of with a list of energy for each band private void GetEnergyList(int B, ref List<double>[] instantEnergyList) { for (int i = 0; i < B; i++) { instantEnergyList[i] = new List<double>(); } short[] samples = soundData.Samples; float timePerSample = 1 / (float)soundData.SampleRate; int sampleIndex = 0; int nextSamples = 1024; int samplesPerBand = nextSamples / B; // for the whole song while (sampleIndex + nextSamples < samples.Length) { complex[] FFT = FastFourier.Calculate(getRange(sampleIndex, nextSamples + sampleIndex, samples)); // foreach band for (int i = 0; i < B; i++) { double energy = 0; for (int j = 0; j < samplesPerBand; j++) energy += FFT[i * samplesPerBand + j].GetMagnitude(); energy /= samplesPerBand; instantEnergyList[i].Add(energy); } if (sampleIndex + nextSamples >= samples.Length) nextSamples = samples.Length - sampleIndex - 1; sampleIndex += nextSamples; samplesPerBand = nextSamples / B; } } // place the actual markers private void PlaceMarkers(List<double> instantEnergyList, int N, float C) { double timePerSample = 1 / (double)soundData.SampleRate; int index = N; int numInBuffer = index; double historyBuffer = 0; //Fill the history buffer with n * instant energy for (int i = 0; i < index; i++) { historyBuffer += instantEnergyList[i]; } // If instantEnergy / samples in buffer < instantEnergy for the next sample then add beatmarker. while (index + 1 < instantEnergyList.Count) { if(instantEnergyList[index + 1] > (historyBuffer / numInBuffer) * C) beatMarkers.Add((index + 1) * 1024 * timePerSample); historyBuffer -= instantEnergyList[index - numInBuffer]; historyBuffer += instantEnergyList[index + 1]; index++; } } } For some reason it's only detecting beats from 637 sec to around 641 sec, and I have no idea why. I know the beats are being inserted from multiple bands since I am finding duplicates, and it seems that it's assigning a beat to each instant energy value in between those values. It's modeled after this: http://www.flipcode.com/misc/BeatDetectionAlgorithms.pdf So why won't the beats register properly?

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