The periodic continuation of a sequence of samples leads to sampling
of the periodic spectrum arising from the segment of the time signal. Due to
the periodic spectrum, N spectral values suffice for the characterzation of
N periodically continued samples. It is to be noted that spectral values are
calculated belonging to the evaluated time segment. This is the so called
short-time spectrum. Even when this segment olitime behaves spectrally like
a function extending indefinitely in time, the convolution with the specture
of the windowing function will change the spectral values.