calculateAutocorrelation() [2/2]

float fl::audio::detector::Pitch::calculateAutocorrelation ( const i16 * pcm, size numSamples )

private

Definition at line 108 of file pitch.cpp.hpp.

                                                                                  {
    // Clear and resize autocorrelation buffer
    mAutocorrelation.clear();
    mAutocorrelation.resize(static_cast<size>(mMaxPeriod + 1), 0.0f);
 
    // Normalize input to float range [-1, 1]
    const float normFactor = 1.0f / 32768.0f;
 
    // Calculate autocorrelation for all lags in the period range
    // ACF[k] = sum(signal[n] * signal[n + k]) for all valid n
    for (int lag = mMinPeriod; lag <= mMaxPeriod; lag++) {
        float sum = 0.0f;
        int validSamples = 0;
 
        for (size i = 0; i + lag < numSamples; i++) {
            float s1 = static_cast<float>(pcm[i]) * normFactor;
            float s2 = static_cast<float>(pcm[i + lag]) * normFactor;
            sum += s1 * s2;
            validSamples++;
        }
 
        // Normalize by number of samples
        if (validSamples > 0) {
            mAutocorrelation[static_cast<size>(lag)] = sum / static_cast<float>(validSamples);
        }
    }
 
    // Find the lag with maximum autocorrelation (best period match)
    int bestLag = findBestPeakLag(mAutocorrelation);
 
    // Calculate confidence based on autocorrelation peak
    if (bestLag > 0) {
        mConfidence = calculateConfidence(mAutocorrelation, bestLag);
 
        // Convert period (lag) to frequency
        return periodToFrequency(bestLag);
    }
 
    // No reliable pitch found
    mConfidence = 0.0f;
    return 0.0f;
}

References calculateConfidence(), findBestPeakLag(), fl::FL_NO_EXCEPT, mAutocorrelation, mConfidence, mMaxPeriod, mMinPeriod, and periodToFrequency().

Referenced by update().

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