Method calc_fft

DSView/pv/data/spectrumstack.cpp:134–186

}

void SpectrumStack::calc_fft()
{
    _spectrum_state = Running;
    // Get the dso data
    pv::data::DsoSnapshot *data = NULL;
    pv::view::DsoSignal *dsoSig = NULL;

    for(auto s : _session->get_signals()) {
        if (s->signal_type() == SR_CHANNEL_DSO) {
            dsoSig = (view::DsoSignal*)s;
            if (dsoSig->get_index() == _index && dsoSig->enabled()) {
                data = dsoSig->data();
                break;
            }
        }
    }

    if (data == NULL || data->empty())
        return;

    if (data->get_sample_count() < _sample_num * _sample_interval)
        return;

    // Get the samplerate
    _samplerate = data->samplerate();
    if (_samplerate == 0.0)
        _samplerate = 1.0;

    // prepare _xn data
    const int offset = dsoSig->get_hw_offset();
    const double vscale = dsoSig->get_vDialValue() * dsoSig->get_factor() * DS_CONF_DSO_VDIVS / (1000*255.0);
    const uint16_t step = _sample_interval;
    const uint8_t *const samples = data->get_samples(0, _sample_num*_sample_interval-1, _index);
    double wsum = 0;
    
    for (unsigned int i = 0; i < _sample_num; i++) {
        double w = window(i, _windows_index);
        _xn[i] = (samples[i*step] - offset) * vscale * w;
        wsum += w;
    }

    // fft
    fftw_execute(_fft_plan);

    // calculate power spectrum
    _power_spectrum[0] = abs(_xk[0])/wsum;  /* DC component */
    for (unsigned int k = 1; k < (_sample_num + 1) / 2; ++k)  /* (k < N/2 rounded up) */
         _power_spectrum[k] = sqrt((_xk[k]*_xk[k] + _xk[_sample_num-k]*_xk[_sample_num-k]) * 2) / wsum;
    if (_sample_num % 2 == 0) /* N is even */
         _power_spectrum[_sample_num/2] = abs(_xk[_sample_num/2])/wsum;  /* Nyquist freq. */

    _spectrum_state = Stopped;
}

double SpectrumStack::window(uint64_t i, int type)
{