### FFT and IFFT Blocks Data Order

The FFT block enables you to output the frequency indices in
linear or bit-reversed order. Because linear ordering of the frequency
indices requires a bit-reversal operation, the FFT block may run more
quickly when the output frequencies are in bit-reversed order.

The input to the IFFT block can be in linear or bit-reversed
order. Therefore, you do not have to alter the ordering of your data
before transforming it back into the time domain. However, the IFFT
block may run more quickly when the input is provided in bit-reversed
order.

### Find the Bit-Reversed Order of Your Frequency Indices

Two numbers are bit-reversed values of each other when the binary
representation of one is the mirror image of the binary representation
of the other. For example, in a three-bit system, one and four are
bit-reversed values of each other, since the three-bit binary representation
of one, 001, is the mirror image of the three-bit binary representation
of four, 100. In the diagram below, the frequency indices are in linear
order. To put them in bit-reversed order

Translate the indices into their binary
representation with the minimum number of bits. In this example, the
minimum number of bits is three because the binary representation
of 7 is 111.

Find the mirror image of each binary
entry, and write it beside the original binary representation.

Translate the indices back to their
decimal representation.

The frequency indices are now in bit-reversed order.

The next diagram illustrates the linear and bit-reversed outputs
of the FFT block. The output values are the same, but they appear
in different order.