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## Zero Padding Theorem

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:Let and define . Then with . Denote the original frequency index by , where and the new frequency index by , where .

Definition:Theideal bandlimited interpolationof a spectrum , , to an arbitrary new frequency is defined as

Note that this is just the definition of the DFT with replaced by . That is, the spectrum is interpolated by projecting onto the new sinusoid exactly as if it were a DFT sinusoid. This makes the most sense when is assumed to be samples of atime-limitedsignal. That is, if the signal really is zero outside of the time interval, then the inner product between it and any sinusoid will be exactly as in the equation above. Thus, for time limited signals, this kind of interpolation is ideal.

Definition:Theinterpolation operatorinterpolates a signal by an integer factor. That is,

Since is initially only defined over the roots of unity, while is defined over roots of unity, we define for by ideal bandlimited interpolation.

Theorem:For any

Proof:Let with . Then