A special circuit is necessary for the data input of the first stage. An incoming
data stream of N pieces of data must be divided into two paths of N/2
pieces of data each. Due to division into two parallel paths, the clock rate
must be halved. The circuit required for this is a demultiplexed followed by
a FIFO register (FIFO-first 1n, first out). A special structure is shown in
Figure 7.3.9 that carries out the splitting of the data with synchronous clocking
and minimal storage capacity [107]. In the illustration, the delays are defined
relative to the input clock. The illustration characterizes the general
procedure. It does not specify the exact technical implementation.
The entire FFT processor resulting from vertical projection is shown in
Figure 7.3.10. The configuration consists of N PEs. Delay commutators are
located between the PEs. Due to the continued halving, their control signals
are extracted using frequency dividers. This example shows clearly that
solving the data input leads to considerable costs in term of registers and control
circuitry.