Flip Operator

We define the flip operator by

$$\hbox{\sc Flip}_n(x) \isdef x(-n) \protect$$ (7.1)

which, by modulo indexing, is also $x(N-n)$. The $\hbox{\sc Flip}()$ operator reverses the order of samples $1$ through $N-1$ of a sequence, leaving sample 0 alone, as shown in Fig. 7.1a. Thanks to modulo indexing, it can also be viewed as ``flipping'' the sequence about the vertical axis, as shown in Fig. 7.1b. The interpretation of Fig. 7.1b is usually the one we want, and the $\hbox{\sc Flip}$ operator is usually thought of as ``time reversal'' when applied to a signal $x$ or ``frequency reversal'' when applied to a spectrum $X$.

Figure: Illustration of $x$ and $\hbox{\sc Flip}(x)$ for $N=5$ and two domain interpretations: a) $n\in [0,N-1]$. b) $n\in [-(N-1)/2, (N-1)/2]$.