The Discrete Fourier Transform (DFT) provides a signal representation in terms of all the frequencies present in the signal. Often, however, we are interested only in certain frequencies or in a range of frequencies. Discrete-time filters can provide a simple and computationally efficient way of isolating ranges of frequencies. Consider, for example, the case where we want to remove the DC value from a signal. We could take the DFT of the signal, replace the zero-frequency component with zero, and then take the inverse DFT. A much simpler route would be to implement the filter with an input/output relationship.