How I can set frequency band to my array from KissFFT? Sampling frequency is 44100 and I need to set it to my array realPartFFT. I have no idea, how it works. I need to plot my spectrum chart to see if it counts right. When I plot it now, it still has only 513 numbers on the x axis, without the specified frequency.
int windowCount = 1024;
float floatArray[windowCount], realPartFFT[(windowCount / 2) + 1];
kiss_fftr_cfg cfg = kiss_fftr_alloc(windowCount, 0, NULL, NULL);
kiss_fft_cpx cpx[(windowCount / 2) + 1];
kiss_fftr(cfg, floatArray, cpx);
for (int i = 0; i < (windowCount / 2) + 1; ++)
realPartFFT[i] = sqrtf(powf(cpx[i].r, 2.0) + powf(cpx[i].i, 2.0));
First of all: KissFFT doesn't know anything about the source of the data. You pass it an array of real numbers of a given size N, and you get in return an array of complex values of size N/2+1. The input array may be the whether forecast of the next N hours of the number of sunspots of the past N days. KissFFT doesn't care.
The mapping back to the real world needs to be done by you, so you have to interpret the data. As of you code snippet, you are passing 1024 of floats (I assume that floatArray contains the input data). You then get back an array of 513 (=1024/2+1) pairs of floats.
If you are sampling with 44.1 KHz and pass KissFFT chunks of 1024 (your window size) samples, you will get as highest frequency 22.05 KHz and as lowest frequency about 43 Hz (44,100 / 1024). You can get even lower by passing bigger chunks to KissFFT, but keep in mind that processing time will grow (with the fourth power of N, IIRC)!
Btw: You may consider making your windowSize variable const, to allow the compiler do some optimizations. Optimizations are very valuable when doing number crunching. In this case the effect may be negligible, but it's a good starting point.