Fast Fourier Transform - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 233

// FFTr2.c FFT using TI's optimized FFT function and real-time input

#include "dsk6713_aic23.h" //codec-DSK support file
Uint32 fs=DSK6713_AIC23_FREQ_8KHZ; //set sampling rate
#include <math.h>
#define N 256 //number of FFT points

#define RADIX 2 //radix or base

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

#define PI 3.14159265358979
short i = 0;
short iTwid[N/2]; //index for twiddle constants W
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 on boundary
#pragma DATA_ALIGN(x,sizeof(Complex)) //align input x on boundary

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

//bit reverse W

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

for(i=0; i<N; i++)

Xmag[i] = 0; //init output magnitude

while (1) //infinite loop

{

output_sample(32000); //negative spike for reference

for( i = 0 ; i < N ; i++ )
{
x[i].re = (float)((short)input_sample()); //external input
x[i].im = 0.0 ; //zero imaginary part
if(i>0) output_sample((short)Xmag[i]);//output magnitude
}
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)/32; //magnitude of x
}
}

FIGURE 6.17. FFT program of real-time input using TI's optimized radix-2 FFT function
( FFTr2.c ).