Clarification on Matlab function "Interpft".

Hello there,

My problem is quite simple. I've been trying to up-sample a time domain signal by exploiting the FFT operator. What I did is quite simple:

1. FFT to map the signal from Time Domain to Frequency Domain;
2. FFTSHIFT to center wrt the 0-Frequency bin;
3. Padding both sides with an equal amount of zeros;
4. IFFTSHIFT;
5. IFFT.

The results that I got from the aforementioned algorithm were then compared with the ones carried out by the "INTERPFT" operator. What I found out is that the results are equal when the size of the input sequence is odd. Vice-versa, when the size is even, the results differ.

Then, I checked the "INTERPFT" code in order to understand my mistake. As far as I understood, I put the zeros in the wrong place and I did not take care of the sample at (- Sampling Frq.)/2.

See below the chunk of the code from "INTERPFT":

a = fft(x,[],1);
nyqst = ceil((m+1)/2);
b = [a(1:nyqst,:) ; zeros(ny-m,n) ; a(nyqst+1:m,:)];
if rem(m,2) == 0
    b(nyqst,:) = b(nyqst,:)/2;
    b(nyqst+ny-m,:) = b(nyqst,:);
end

Can anybody clarify on these steps? Why is it needed to split the energy related to the Frequency bin at (- Sampling Frq.)/2?

Thanks for your time,

Emiliano