Zero Padding Theorem (Spectral Interpolation)

A fundamental tool in practical spectrum analysis is zero padding. This theorem shows that zero padding in the time domain corresponds to ideal interpolation in the frequency domain (for truly time-limited signals):

Theorem: For any $x\in{\bf C}^N$

$$\zbox {\hbox{\sc ZeroPad}_{LN}(x) \leftrightarrow \hbox{\sc Interp}_L(X)}$$

where $\hbox{\sc ZeroPad}()$ was defined in Eq. (7.3), followed by the definition of $\hbox{\sc Interp}()$.

Proof: Let $M=LN$ with $L\geq 1$. Then

$$\begin{eqnarray*} \hbox{\sc DFT}_{M,k^\prime }(\hbox{\sc ZeroPad}_M(x)) &=& \su... ...M} \\ &\isdef & X(\omega_{k^\prime }) = \hbox{\sc Interp}_L(X). \end{eqnarray*}$$

Thus, this theorem follows directly from the definition of the ideal interpolation operator $\hbox{\sc Interp}()$. See §8.1.3 for an example of zero-padding in spectrum analysis.