This results in four cases. Quite naturally, the frequency domain
has the same four cases,
discrete or continuous in frequency, and
finite or infinite in bandwidth.
When time is discrete, the frequency axis is finite, and vice versa.
This book has been concerned almost exclusively with the
discrete-time, discrete-frequency case (the DFT), and in that case,
both the time and frequency axes are finite in length. In the
following sections, we briefly summarize the other three
cases. Table B.1 summarizes all four Fourier-transform cases
corresponding to discrete or continuous time and/or frequency.
Four cases of sampled/continuous
time and frequency.