The position of the PEs during vertical projection is obvious. However,
special circuitry between the PEs is required to prepare the correct data input.
From stage to stage the length of the sequence onto which the FFT is applied
is halved. Given that the previous stage led to a DPT of length 2n, in accordance
with the perfect shuffle, the sequence of length 2n must be halved, and
the first and the (n.+ l )th values must be fed to the following PE. Then the second
and the (n+2)th values are fed to it. The required data sorting is illustrated
in Figure 7.3.6. In the illustration, the fact that the sequence must be
by n clock cycles in accordance with the position of the midpoint is
also accounted for.