Function makeSyntheticVowel

tests/fl/audio/test_helpers.h:227–251

Generate a synthetic vowel using additive synthesis with Gaussian formant envelopes Simplest physically-motivated voice model: harmonic series with 1/h rolloff shaped by two resonance peaks

/// Generate a synthetic vowel using additive synthesis with Gaussian formant envelopes
/// Simplest physically-motivated voice model: harmonic series with 1/h rolloff shaped by two resonance peaks
inline Sample makeSyntheticVowel(float f0, float f1Freq, float f2Freq,
                                       u32 timestamp, float amplitude = 16000.0f,
                                       int count = 512, float sampleRate = 44100.0f) {
    fl::vector<fl::i16> data(count, 0);
    const float f1Bw = 150.0f; // F1 bandwidth
    const float f2Bw = 200.0f; // F2 bandwidth
    const float maxFreq = fl::min(4000.0f, sampleRate / 2.0f);

    for (int h = 1; h * f0 < maxFreq; ++h) {
        float freq = f0 * h;
        // Natural 1/h rolloff
        float naturalAmp = 1.0f / static_cast<float>(h);
        // Gaussian formant envelopes
        float f1Gain = fl::expf(-0.5f * (freq - f1Freq) * (freq - f1Freq) / (f1Bw * f1Bw));
        float f2Gain = fl::expf(-0.5f * (freq - f2Freq) * (freq - f2Freq) / (f2Bw * f2Bw));
        float formantGain = fl::max(f1Gain, f2Gain);
        float harmonicAmp = amplitude * naturalAmp * fl::max(0.05f, formantGain);

        for (int i = 0; i < count; ++i) {
            float phase = 2.0f * FL_M_PI * freq * i / sampleRate;
            data[i] += static_cast<fl::i16>(harmonicAmp * fl::sinf(phase));
        }
    }
    return Sample(data, timestamp);
}

/// Generate a jittered vowel: synthetic vowel with per-period amplitude and timing jitter
/// Simulates real vocal cord irregularity — higher envelope jitter and lower autocorrelation