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C++ Complex Number — 02

Date: 2023-03-10Last modified: 2024-11-02
C++complexfft
CC-BY-SA-3.0 diagram by Daniele Pugliesi on Wikimedia

Table of contents

  const double Fs = 1000.0;                       // Sampling frequency (Hz)
  const int N = 1024;                             // signal length
  std::vector<double> signal(N);                  // time-domain signal
  std::vector<std::complex<double>> spectrum(N);  // frequency-domain spectrum

  // fill signal with data...
  for (int n = 0; n < N; n++) {
    signal[n] = 10 * std::sin(2 * M_PI * 25 * n / Fs) +
                20 * std::sin(2 * M_PI * 50 * n / Fs) +
                30 * std::sin(2 * M_PI * 100 * n / Fs);
  }

  // create FFTW plan
  fftw_plan plan = fftw_plan_dft_r2c_1d(
      N, signal.data(), reinterpret_cast<fftw_complex*>(spectrum.data()),
      FFTW_ESTIMATE);

  // perform FFT
  fftw_execute(plan);

  // destroy plan
  fftw_destroy_plan(plan);

  for (int i = 0; i < N / 2 + 1; i++) {
    auto norm = abs(spectrum[i]);
    if (norm > 3000) {
      double freq = i * Fs / N;
      fmt::print(
          "Freq: {:5.1f}   Norm: {:7.1f}   Complex: {:7.1f} + {:7.1f}i\n", freq,
          norm, spectrum[i].real(), spectrum[i].imag());
    }
  }

Possible output

Freq:  25.4   Norm:  3741.2   Complex: -3524.0 + -1256.2i
Freq:  49.8   Norm:  9643.3   Complex:  5698.9 + -7779.3i
Freq:  98.6   Norm:  3290.9   Complex:  3138.5 +  -989.8i
Freq:  99.6   Norm: 11594.9   Complex: 11036.1 + -3556.3i
Freq: 100.6   Norm:  7779.4   Complex: -7390.0 +  2430.3i

References