calc_fourier.c

/*--------------------------------------------------------------------------------

離散フーリエ変換と逆離散フーリエ変換

discrete Fourier transform and inverse discrete Fourier transform

calc_fourier.c

*--------------------------------------------------------------------------------*/

#include "DFT.h"

// 離散フーリエ変換

// データxを入力し、フーリエ係数Fを出力

// discrete Fourier transform

// input the data x, output the Fourier coefficient F

void fourier( int num, complex *x, complex *F )

{

int k, n;

double angle;

double real_temp;

double imag_temp;

for( k = 0; k < num; k++ ){

real_temp = 0.0;

imag_temp = 0.0;

for( n = 0; n < num; n++ ){

angle = (2.0 * M_PI * n * k)/(double)num;

// 実部の計算

// calculation of the real part

real_temp += (  x[n].real * cos(angle) + x[n].imag * sin(angle) );

// 虚部の計算

// calculation of the imaginaryl part

imag_temp += ( -x[n].real * sin(angle) + x[n].imag * cos(angle) );

}

F[k].real = real_temp;

F[k].imag = imag_temp;

}

}

/*----------------------------------------------------------------------*/

// 逆離散フーリエ変換

// フーリエ係数Fを入力し、データxを出力

// inverse discrete Fourier transform

// input the Fourier coefficient F, and output the data x

void inv_fourier( int num, complex *x, complex *F )

{

int k, n;

double angle;

double real_temp;

double imag_temp;

for( n = 0; n < num; n++ ){

real_temp = 0.0;

imag_temp = 0.0;

for( k = 0; k < num; k++ ){

angle = (2.0 * M_PI * n * k)/(double)num;

// 実部の計算

// calculation of the real part

real_temp += ( F[k].real * cos(angle) - F[k].imag * sin(angle) );

// 虚部の計算

// calculation of the imaginaryl part

imag_temp += ( F[k].real * sin(angle) + F[k].imag * cos(angle) );

}

x[n].real = real_temp/(double)num;

x[n].imag = imag_temp/(double)num;

}

}