Verifying FFT Result using DSPLIB Function.

Hi Guys,

I have been stuck with this FFT stuff since long and still I am unable to verify the result. I am simply using dsk_app.c project for C6713 board and implementing DSPLIB function DSPF_sp_cfftr2_dit to compute 1024 point FFT over 1024 complex pairs of input. I am going to explain over all steps which has been a part of my implementation. Please have a look if I am wrong any where.

I defined these parts in dsk_app.c project.

#define BUFFSIZE 1024                                // no. of FFT points (1024)
#pragma DATA_ALIGN(fft_buffer, 8)
float fft_buffer[2*BUFFSIZE];                         // Length of input data (2048)
#pragma DATA_ALIGN(twiddles, 8)
float twiddles[BUFFSIZE];                             // twiddles of FFT length (1024)
float fft_mag[BUFFSIZE];
short bitrev_table[BUFFSIZE];                      // bitrev_table of FFT length (1024)

Following parts are already defined in dsk_app.c project as follows.

Int16 gBufferXmtPing[BUFFSIZE];  // Transmit PING buffer
Int16 gBufferXmtPong[BUFFSIZE];  // Transmit PONG buffer
Int16 gBufferRcvPing[BUFFSIZE];  // Receive PING buffer
Int16 gBufferRcvPong[BUFFSIZE];  // Receive PONG buffer

In main(), I generated coefficient table.

fft_r2_init(twiddles, BUFFSIZE, bitrev_table);

void fft_r2_init(float *w, int n, short *br_table)
{
    int i;
    gen_twiddle(w, n);        // generate twiddle table of n/2 complex twiddles
    //bit_rev(w, n/2);        // put twiddle table in bit-reversed order
    DSPF_sp_bitrev_cplx((double *)w, br_table, n/2);
    for(i=0; i<n; i++)        // initialize bit-rev table to zero
    br_table[i] = 0;
    bitrev_index(br_table, n);
}

-processBuffer() -  I processed audio data with in this function once it received.

For PING Buffer

Here attached plot of gBufferRcvPing after getting 1024 samples of input. It looks fine to me.

http://i31.tinypic.com/35lcc2a.jpg

 if (pingPong == PING) {

  //The DSPf algorithms expect the real data in the even input buffer locations (2*k) and imaginary data in odd location (2*k+1)

        for(k=0; k < 2*BUFFSIZE; k++)

            {
                fft_buffer[2*k] = (float)gBufferRcvPing[k]; //real
                fft_buffer[2*k+1] = 0.0; //imag
            }

Called FFT and bit reversed function.

fft_r2(fft_buffer, twiddles, BUFFSIZE, bitrev_table);

void fft_r2(float *x, float *w, int n, short *br_table)
{
    DSPF_sp_cfftr2_dit(x, w, n);    // in-place FFT

    DSPF_sp_bitrev_cplx((double *)x, br_table, n);

}

Here attached plot of fft_buffer after FFT and bit reversal.

http://i31.tinypic.com/dq6cub.jpg

Then calculated FFT magnitude and copied it back to gBufferRcvPing buffer.

            for(l=0; l < BUFFSIZE; l++)
    {
     fft_mag[l] = sqrt(fft_buffer[2*l]*fft_buffer[2*l] + fft_buffer[2*l+1]*fft_buffer[2*l+1]);

     gBufferRcvPong[l] = (Int16)fft_mag[l];
    }

  /* Copy receive PING buffer to transmit PING buffer */
 copyData(gBufferRcvPing, gBufferXmtPing, BUFFSIZE);

void copyData(Int16 *inbuf, Int16 *outbuf, Int16 length)
{
    Int16 i = 0;

    for (i = 0; i < length; i++) {
        outbuf[i]  = inbuf[i];
    }
}

Here attached plot of fft_mag after calculating magnitude of FFT transform.

http://i30.tinypic.com/15wntlg.jpg

I used the same approach for PONG Buffer.

Here attached plot of fft_buffer after FFT and bit reversal for PONG buffer.

http://i28.tinypic.com/acebv6.jpg

and here find the plot of fft_mag after calculating magnitude of FFT transform for PONG buffer.

http://i28.tinypic.com/2zemoo1.png

Moreoever, I am not sure how much is the sampling frequency, default one is 48Khz for dsk_app.c project, I changed the following parameter with in aic23.h file to make it 8Khz but I don't know wheather its working fine or not.

#define AIC23_DEFAULTPARAMS {            

  AIC23_REG8_8KHZ,                         }

Is this this the right way to change Sampling Rate ?

Where am I doing any thing wrong while calculating fft and magnitude ?

Why fft_mag doesn't show one clear peak for PING and PONG register ?

Is there any thing wrong with the length of twiddles, BUFFSIZE and fft_buffer ?

Please help me out.

Thanks.