Circular Convolution Using DFT and IDFT
The convolution theorem for the discrete-time Fourier transform (DTFT) indicates that a convolution of two sequences can be obtained as the inverse transform of the product of the individual transforms. An important simplification occurs when one of sequences is N-periodic, denoted here by yN because DTFT{yN} is non-zero at only discrete frequencies, and therefore so is its product with the continuous function DTFT{x} That leads to a considerable simplification of the inverse transform.
Then the convolution can be written as:
The step-by-step values for the provided input is shown at the bottom of the page: