Let denote any continuous-time signal having a continuousFourier transform

Let

denote the samples of at uniform intervals of seconds.
Then can be exactly reconstructed from its samples if
for all
.^{D.3}

Proof: From the continuous-time aliasing theorem (§D.2), we
have that the discrete-time spectrum
can be written in
terms of the continuous-time spectrum
as

where
is the ``digital frequency'' variable.
If
for all
, then the above
infinite sum reduces to one term, the term, and we have

At this point, we can see that the spectrum of the sampled signal
coincides with the nonzero spectrum of the continuous-time
signal . In other words, the DTFT of is equal to the
FT of between plus and minus half the sampling rate, and the FT
is zero outside that range. This makes it clear that spectral
information is preserved, so it should now be possible to go from the
samples back to the continuous waveform without error, which we now
pursue.

To reconstruct from its samples , we may simply take
the inverse Fourier transform of the zero-extended DTFT, because

By expanding
as the DTFT of the samples , the
formula for reconstructing as a superposition of weighted sinc
functions is obtained (depicted in Fig.D.1):

The ``sinc function'' is defined with in its argument so that it
has zero crossings on the nonzero integers, and its peak magnitude is
1. Figure D.2 illustrates the appearance of the sinc function.

We have shown that when is bandlimited to less than half the
sampling rate, the IFT of the zero-extended DTFT of its samples
gives back the original continuous-time signal .
This completes the proof of the
sampling theorem.

Conversely, if can be reconstructed from its samples
, it must be true that is bandlimited to
, since a sampled signal only supports frequencies up
to (see §D.4 below). While a real digital signal
may have energy at half the sampling rate (frequency ),
the phase is constrained to be either 0 or there, which is why
this frequency had to be excluded from the sampling theorem.

A one-line summary of the essence of the sampling-theorem proof is

where
.

The sampling theorem is easier to show when applied to sampling-rate
conversion in discrete-time, i.e., when simple downsampling of a
discrete time signal is being used to reduce the sampling rate by an
integer factor. In analogy with the continuous-time aliasing theorem
of §D.2, the downsampling theorem (§7.4.11)
states that downsampling a digital signal by an integer factor
produces a digital signal whose spectrum can be calculated by
partitioning the original spectrum into equal blocks and then
summing (aliasing) those blocks. If only one of the blocks is
nonzero, then the original signal at the higher sampling rate is
exactly recoverable.