How do I use vDSP functions for Short Time Fourier Transform? - iphone

Question

I trying to understand how to use vDSP functions for STFT. So I use FFT code from apple's expamles and I can get FFT of first 1024 frames but how could I get FFT of next 1024 - 2047 frames and so on, until the end of file.. (in this case I imagine the size of file is int f = 10000).
//vDSP variables
DOUBLE_COMPLEX_SPLIT A;
FFTSetupD setupReal;
uint32_t log2n;
uint32_t n, nOver2;
int32_t stride;
double *obtainedReal;
double scale;
log2n = N;
n = 1 << log2n;
stride = 1;
nOver2 = n/2;
int f = 10000;
buffer = malloc(f *sizeof(double));
obtainedReal = malloc(f *sizeof(double));
A.realp = malloc(f *sizeof(double));
A.imagp = malloc(f *sizeof(double));
vDSP_ctozD((DOUBLE_COMPLEX*) buffer, 2, &A, 1, nOver2);
setupReal = vDSP_create_fftsetupD(log2n, FFT_RADIX2);
if (setupReal == NULL) {
NSLog(#"fft_setup failed to allocate enough memory for real FFT\n");
return 0 ;
}
vDSP_fft_zripD(setupReal, &A, stride, log2n, FFT_FORWARD);
scale = (double) 1.0 / (2 * n);
vDSP_vsmulD(A.realp, 1, &scale, A.realp, 1, nOver2);
vDSP_vsmulD(A.imagp, 1, &scale, A.imagp, 1, nOver2);
vDSP_ztocD(&A, 1, (DOUBLE_COMPLEX *) obtainedReal, 2, nOver2);

Answer 1

If you simply want the FFT of the next 1024 elements, add nOver2 to A.realp and to A.imagp, then perform another vDSP_fft_zripD and another vDSP_ztocD. You will probably want to advance obtainedReal too, or the new results will overwrite the old results.
Note that changing A.realp and A.imagp loses the starting addresses, so you will not be able to free this memory unless you recalculate the starting addresses or save them elsewhere before changing A.realp and A.imagp.
Also, 10,000 is not an integer multiple of 1024, so your last portion will not have 1024 elements, so you need to figure out an alternative, such as getting more data or padding the data with zeroes.
You are allocating too much memory for A.realp and A.imagp. Each of them receives half of the elements in buffer, so each of them only needs half as much memory.
Even that much memory is not needed. You can use vDSP_ctozD to move just 1024 elements into A.realp and A.imagp (512 each), then perform an FFT, then move the data to obtainedReal using vDSP_ztocD, then move on to the next group by using vDSP_ctozD to move 1024 new elements into the same space in A.realp and A.imagp that was used before.