I implemented an interpolation method in C++ based on this equation
y(t) = x[n]*sinc((t-nT)/T) where: n - index of input signal, x - input signal buffer, t - is an actual time, nT - time of input samples, T - period of input signal.
I calculated it on 33 samples:1+2*16. I tested it on Matlab and C++ on VS 2010. It works great on close frequencies (eg. 24kHz to 20kHz) but when I tested situation where the input signal is 24kHz and has a peak on 8kHz and then I wanted to interpolated it to 700Hz it gave me a peak at 300Hz so it is totally wrong. I expected that the main couse of this situation is that I did not use the cutoff frequency for low pass filter in this equation above. Seriously, I do not know how to add low pass filtration to my method.
Thanks for all your suggestions or advise.