Forums

fftw problem

Started by bauzane June 5, 2013
I'm an Italian university student. I'm trying to use FFTW libraries. I have problems but I don't know in which forum I may ask help so I try here. I wrote the following C program

#include
#include
#include
#include

void main()
{
int i;
int N, num_points; // dimensione trasformata
double T;
double *in; // puntatore dati in ingresso alla FFT
fftw_complex *out, *in2, *out2; // puntatore ai dati in uscita della FFT
fftw_plan trasf_diretta, trasf_inversa;

N=8;
num_points=N/2+1;
T=0.08;

// allocazione memoria
in = (double *) malloc(sizeof(double)*N);
out = (fftw_complex *) fftw_malloc(sizeof(fftw_complex)*num_points);

in[0]0000.0;
in[1]000.0;
in[2]`000.0;
in[3]0000.0;
in[4]=0.0;
in[5]=0.0;
in[6]=0.0;
in[7]=0.0;

// ffttw_plan_dft_r2c_1d (dimensione, segnale, risultato, flag ottimizzazione)
trasf_diretta = fftw_plan_dft_r2c_1d (N, in, out, FFTW_ESTIMATE);

fftw_execute(trasf_diretta);

for(i=0;i {
out[i]=out[i]*T;
}

for(i=0;i {
printf(" %d %lf %lf\n", i, creal(out[i]), cimag(out[i]));
}
// antitrasformazione
out2 = (double *) malloc(sizeof(double)*N);
trasf_inversa = fftw_plan_dft_c2r_1d (N, out, out2, FFTW_ESTIMATE);
fftw_execute(trasf_inversa);

for (i=0; i {
printf(" %d %lf\n", i, out2[i]/N/T);
}

fftw_destroy_plan(trasf_diretta);
fftw_destroy_plan(trasf_inversa);
fftw_free(in);
fftw_free(out);
fftw_free(out2);
}

The program compute forward FFT correctly but not inverse FFT because generate a ifft compressed signal along x axis. I can't understand where the problem is. Can anyone help me? Thank You Roberto
Hi,
Check what the line "out[i]=out[i]*T;" is actually doing. I dont know how the operator "*" for fftw_complex* is defined. Also, T is double, so I guess T is actually multiplying only the first double (real part) of the complex out. It would distort the phase of the signal yielding weird things like shrinking in time domain.
Best,
- Felipe
I'm an Italian university student. I'm trying to use FFTW libraries. I have problems but I don't know in which forum I may ask help so I try here. I wrote the following C program
>
>#include
>#include
>#include
>#include
>
>void main()
>{
>int i;
>int N, num_points; // dimensione trasformata
>double T;
>double *in; // puntatore dati in ingresso alla FFT
>fftw_complex *out, *in2, *out2; // puntatore ai dati in uscita della FFT
>fftw_plan trasf_diretta, trasf_inversa;
>
>N=8;
>num_points=N/2+1;
>T=0.08;
>
>// allocazione memoria
>in = (double *) malloc(sizeof(double)*N);
>out = (fftw_complex *) fftw_malloc(sizeof(fftw_complex)*num_points);
>
>in[0]0000.0;
>in[1]000.0;
>in[2]`000.0;
>in[3]0000.0;
>in[4]=0.0;
>in[5]=0.0;
>in[6]=0.0;
>in[7]=0.0;
>
>// ffttw_plan_dft_r2c_1d (dimensione, segnale, risultato, flag ottimizzazione)
>trasf_diretta = fftw_plan_dft_r2c_1d (N, in, out, FFTW_ESTIMATE);
>
>fftw_execute(trasf_diretta);
>
>for(i=0;i