function sig = ifft_rec(spec, N)
    if(size(spec)<N)
        sig(length(spec)+1 : N) = zeros(1,N-length(spec));
    end;
    if(size(spec) == 1)
        sig = spec(1);
        return;
    end
    odd_spec = spec(1:2:N);
    even_spec = spec(2:2:N);
    a = ifft_rec(odd_spec, N/2);
    b = ifft_rec(even_spec, N/2);
    WN = exp(-j*2*pi/N);
    k = 0:N/2-1;
    sig (1:N/2)= a + (WN.^(-k)).*b;
    sig (N/2+1: N) = a - (WN.^(-k)).*b;