This Demonstration shows impulse and magnitude responses of ideal -band lowpass discrete filters for . While such filters are not realizable in practice, they serve as a desired template for passing certain frequencies while blocking others. The impulse response of an ideal lowpass filter is a sinc sequence (of unit norm in the figure), while its magnitude response is constant in the passband.
An ideal -band discrete filter is a non-realizable filter whose magnitude response takes a single nonzero value in its passband. For example, an ideal lowpass filter passes frequencies below some cut-off frequency and blocks the others; its passband is thus the interval .
The impulse response of an ideal lowpass -band filter is a sinc sequence; its unit-norm version is
The impulse response at is thus (see figure).
The magnitude response of a unit-norm is (see figure)
for , and 0 otherwise.
 M. Vetterli, J. Kovačević, and V. K. Goyal, Foundations of Signal Processing, Cambridge: Cambridge University Press, 2014. www.fourierandwavelets.org.