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Zero padding consists of extending a signal (or spectrum) with zeros to extend its time (or frequency band) limits.

A finer sampling of a frequency spectrum (more samples between frequency 0 and 0.5) can be obtained by padding the time domain signal with zeros before taking the DFT. Duality allows this to work in the opposite direction. If we want a finer sampling in the time domain (interpolation), pad the frequency spectrum with zeros before taking the Inverse DFT.

References:
1. http://books.w3k.org/html/mdft/Zero_Padding.html
2. http://www.dspguide.com/ch10/4.htm

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