Function makeJitteredVowel

tests/fl/audio/test_helpers.h:256–295

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

/// Simulates real vocal cord irregularity — higher envelope jitter and lower autocorrelation
/// than clean synthetic vowels
inline Sample makeJitteredVowel(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);
    fl::fl_random rng(123);
    const float f1Bw = 150.0f;
    const float f2Bw = 200.0f;
    const float maxFreq = fl::min(4000.0f, sampleRate / 2.0f);

    // Generate clean vowel base
    for (int h = 1; h * f0 < maxFreq; ++h) {
        float freq = f0 * h;
        float naturalAmp = 1.0f / static_cast<float>(h);
        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));
        }
    }

    // Apply per-period amplitude jitter (simulates vocal cord irregularity)
    float periodSamples = sampleRate / f0;
    float pos = 0.0f;
    float currentJitter = 1.0f;
    for (int i = 0; i < count; ++i) {
        pos += 1.0f;
        if (pos >= periodSamples) {
            pos -= periodSamples;
            // Random amplitude jitter: +/- 20%
            currentJitter = 0.80f + 0.40f * (static_cast<float>(rng.random16()) / 65535.0f);
        }
        float sample = static_cast<float>(data[i]) * currentJitter;
        data[i] = static_cast<fl::i16>(fl::clamp(sample, -32768.0f, 32767.0f));
    }
    return Sample(data, timestamp);
}

/// Generate a guitar string decay: harmonic series with 1/h^2 rolloff and exponential decay
/// Very periodic waveform — low envelope jitter, high autocorrelation, low ZC CV