calculateAutocorrelationIrregularity()

float fl::audio::detector::Vocal::calculateAutocorrelationIrregularity ( span< const i16 > pcm )

private

Definition at line 407 of file vocal.cpp.hpp.

                                                                     {
    const int n = static_cast<int>(pcm.size());
 
    // Vocal fundamental lag range at mSampleRate
    const int minLag = fl::max(2, mSampleRate / 500);   // 500 Hz
    const int maxLag = fl::min(n / 2, mSampleRate / 172); // 172 Hz
 
    if (minLag >= maxLag || maxLag >= n) return 0.0f;
 
    // Subsample by 4: safe since step(4) << minLag(88).
    // normFactor² cancels in the ratio sum/acf0, so work in raw i16 space.
    const int step = 4;
 
    // ACF[0] = energy (subsampled)
    float acf0 = 0.0f;
    for (int i = 0; i < n; i += step) {
        float s = static_cast<float>(pcm[i]);
        acf0 += s * s;
    }
    if (acf0 < 1.0f) return 0.0f;
 
    // Probe 16 evenly-spaced lags, each with subsampled inner loop.
    // Total: ~16 × (n/4)/lag ≈ 1700 MACs (vs 34K without subsampling).
    const int lagRange = maxLag - minLag;
    const int maxProbes = 16;
    const int lagStep = fl::max(1, lagRange / maxProbes);
    float bestPeak = 0.0f;
    for (int lag = minLag; lag <= maxLag; lag += lagStep) {
        float sum = 0.0f;
        for (int i = 0; i + lag < n; i += step) {
            sum += static_cast<float>(pcm[i]) * static_cast<float>(pcm[i + lag]);
        }
        float normalized = sum / acf0;
        bestPeak = fl::max(bestPeak, normalized);
    }
 
    return 1.0f - bestPeak; // 0 = perfectly periodic, 1 = no periodicity
}

References fl::max(), fl::min(), mSampleRate, fl::span< T, Extent >::size(), and fl::step().

Referenced by update().

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