Setup the accelerator framework for fft on the iPhone

Question

I have set a function to setup the accelerator, after i have read :

Using the Apple FFT and Accelerate Framework

iPhone FFT with Accelerate framework vDSP

and apple docs.

i did this :

void fftSetup()
{

    COMPLEX_SPLIT   A;
    FFTSetup        setupReal;
    uint32_t        log2n;
    uint32_t        n, nOver2;
    int32_t         stride;
    uint32_t        i;
    float          *originalReal, *obtainedReal;
    float           scale;
    uint32_t L = 1024;
    float *mag = new float[L/2];


     log2n = 10 ;
     n = 1 << log2n;
     stride = 1;
     nOver2 = n / 2;

    printf("1D real FFT of length log2 ( %d ) = %d\n\n", n, log2n);

    for (i = 0; i < n; i++)
        originalReal[i] = (float) (i + 1);


    vDSP_ctoz((COMPLEX *) originalReal,2,&A,1,nOver2);

    A.realp = (float *) malloc(nOver2 * sizeof(float));
    A.imagp = (float *) malloc(nOver2 * sizeof(float));

    setupReal = vDSP_create_fftsetup(log2n, FFT_RADIX2);

    vDSP_fft_zrip(setupReal, &A, stride, log2n, FFT_FORWARD);
    vDSP_fft_zrip(setupReal, &A, stride, log2n, FFT_INVERSE);




    //get magnitude;
    for(i = 1; i < L/2; i++){
        mag[i] = sqrtf(A.realp[i]*A.realp[i] + A.imagp[i] * A.imagp[i]);
    }





    scale = (float) 1.0 / (2 * n);
    vDSP_vsmul(A.realp, 1, &scale, A.realp, 1, nOver2);
    vDSP_vsmul(A.imagp, 1, &scale, A.imagp, 1, nOver2);


}

questions :

1. my app is always crash with no error(BAD ACCESS) on one of this 2 lines :

originalReal[i] = (float) (i + 1); // or

vDSP_ctoz((COMPLEX *) originalReal,2,&A,1,nOver2);

i guess i did not set a good value for log2n ? (10 to get 1024 window ? )

• how do i get the real magnitude of the bins? my actual fft? the same i wrote here ?

• where do i input MY data buffer array (exactly where in my code ? instead originalReal?)

thanks a lot.

Answer 1

I actually manage to make it work ,when i insert into it a sin wave of a certain f.

This is the code :

   COMPLEX_SPLIT   A;
    FFTSetup        setupReal;
    uint32_t        log2n;
    uint32_t        n, nOver2;
    int32_t         stride;
    uint32_t        i;
    float          *originalReal, *obtainedReal;
    float           scale;
    uint32_t L = 1024;
    float *mag = new float[L/2];


     log2n = 10 ;
     n = 1 << log2n;
     stride = 1;
     nOver2 = n / 2;

    //printf("1D real FFT of length log2 ( %d ) = %d\n\n", n, log2n);

    A.realp = (float *) malloc(nOver2 * sizeof(float));
    A.imagp = (float *) malloc(nOver2 * sizeof(float));
    originalReal = (float *) malloc(n * sizeof(float));
    obtainedReal = (float *) malloc(n * sizeof(float));

     for (i = 0; i < n; i++)
         originalReal[i] = cos(2*3.141592*11000*i/44100);//(float) (i + 1);


    vDSP_ctoz((COMPLEX *) originalReal,2,&A,1,nOver2);



    setupReal = vDSP_create_fftsetup(log2n, FFT_RADIX2);

    vDSP_fft_zrip(setupReal, &A, stride, log2n, FFT_FORWARD);
    //vDSP_fft_zrip(setupReal, &A, stride, log2n, FFT_INVERSE);

    scale = (float) 1.0 / (2 * n);
    vDSP_vsmul(A.realp, 1, &scale, A.realp, 1, nOver2);
    vDSP_vsmul(A.imagp, 1, &scale, A.imagp, 1, nOver2);


    //get magnitude;
    for(i = 1; i < L/2; i++)
    {
        mag[i] = sqrtf(A.realp[i]*A.realp[i] + A.imagp[i] * A.imagp[i]);
           NSLog(@"%d:%f",i,mag[i]);
    }

Actually its not 44hz between bins,as the guy wrote in the post above! but 43 ! 22050/512=43 . this thing is critical ! because in the higher bins- such as bin[300] you get a completely different resault for 44 and 43 ! (its 300hz drift). so take care of that .