Fast Fourier Transform - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 236

Example 6.6: Radix-4 FFT of Real-Time Input Using TI's C-Callable
Optimized FFT Function ( FFTr4 )

This example expands Example 6.5 for a real-time input signal. Figure 6.20 shows
the C source program FFTr4.c that calls a radix-4 FFT function to take the FFT
of a real-time input signal. The same FFT support functions are used in this example

// FFTr4.c FFT using TI's optimized FFT function with 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 // Specifies number of

complex FFT points
unsigned short JIndex[4*N]; //index for digit reversal

unsigned short IIndex[4*N]; //index for digit reversal

int i, count;
float Xmag[N]; //magnitude spectrum of x
typedef struct Complex_tag {float re,im;}Complex;
Complex W[3*N/2];

//array for twiddle constants

Complex x[N]; //N complex data values

double delta = 2*3.14159265359/N;
#pragma DATA_ALIGN(x,sizeof(Complex)); //align x on boundary
#pragma DATA_ALIGN(W,sizeof(Complex)); //align W on boundary

void main()
{
R4DigitRevIndexTableGen(N,&count,IIndex,JIndex);//for digit rev index
for(i = 0; i < 3*N/4; i++)
{

W[i].re = cos(delta*i); //real component of W

W[i].im = sin(delta*i); //Im component of 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 as 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
}
cfftr4_dif(x, W, N); //radix-4 FFT function
digit_reverse((double *)x,IIndex,JIndex,count);//unscramble
for (i =0; i<N; i++)

Xmag[i] = sqrt(x[i].re*x[i].re+x[i].im*x[i].im)/32; //magnitude

}
}

FIGURE 6.20. FFT program that calls TI's optimized radix-4 FFT function using real-time
input ( FFTr4.c ).