Fast Fourier Transform - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 230

// FFTsinetable.c FFT{sine}from table. Calls TI float-point FFT function

#include "dsk6713_aic23.h" //codec-DSK support file
Uint32 fs=DSK6713_AIC23_FREQ_8KHZ; //set sampling rate
#include <math.h>
#define N 32 //number of FFT points

#define FREQ 8 //select # of points/cycle

#define RADIX 2 //radix or base 2

#define DELTA (2*PI)/N //argument for sine/cosine

#define TAB_PTS 32 //# of points in sine_table

#define PI 3.14159265358979
short i = 0;
short iTwid[N/2]; //index for twiddle constants
short iData[N]; //index for bitrev X
float Xmag[N]; //magnitude spectrum of x
typedef struct Complex_tag {float re,im;}Complex;
Complex W[N/RADIX]; //array for twiddle constants
Complex x[N]; //N complex data values
#pragma DATA_ALIGN(W,sizeof(Complex)) //align W
#pragma DATA_ALIGN(x,sizeof(Complex)) //align x

short sine_table[TAB_PTS] = {0,195,383,556,707,831,924,981,1000,
981,924,831,707,556,383,195,-0,-195,-383,-556,-707,-831,-924,-981,
-1000,-981,-924,-831,-707,-556,-383,-195};

void main()
{
for( i = 0 ; i < N/RADIX ; i++ )
{
W[i].re = cos(DELTA*i);

//real component of W

W[i].im = sin(DELTA*i); //neg imag component

} //see cfftr2_dit

for( i = 0 ; i < N ; i++ )
{
x[i].re=3*sine_table[FREQ*i % TAB_PTS]; //wrap when i=TAB_PTS
x[i].im = 0 ; //zero imaginary part

}
digitrev_index(iTwid, N/RADIX, RADIX); //produces index for bitrev() W
bitrev(W, iTwid, N/RADIX);

//bit reverse W

cfftr2_dit(x, W, N ) ; //TI floating-pt complex FFT

digitrev_index(iData, N, RADIX); //produces index for bitrev() X
bitrev(x, iData, N); //freq scrambled->bit-reverse X

for(i = 0 ; i < N ; i++ )
Xmag[i] = sqrt(x[i].re*x[i].re+x[i].im*x[i].im ); //magnitude of X

comm_poll( ) ; //init DSK,codec,McBSP

while (1) //infinite loop

{
output_sample(32000) ; //negative spike as reference

for (i = 1; i < N; i++)
output_sample((short)Xmag[i]); //output magnitude samples
}
}

FIGURE 6.15. FFT program of input data from a table using TI's optimized complex FFT
function ( FFTsinetable.c ).