In signal processing, a sinc filter can refer to either a sinc-in-time filter whose impulse response is a sinc function and whose frequency response is rectangular, or to a sinc-in-frequency filter whose impulse response is rectangular and whose frequency response is a sinc function. Calling them according to which domain the filter resembles a sinc avoids confusion. If the domain is unspecified, sinc-in-time is often assumed, or context hopefully can infer the correct domain.

Sinc-in-time is an ideal filter that removes all frequency components above a given cutoff frequency, without attenuating lower frequencies, and has linear phase response. It may thus be considered a brick-wall filter or rectangular filter.

Its impulse response is a sinc function in the time domain:

$${\displaystyle {\frac {\sin(\pi t)}{\pi t}}}$$

while its frequency response is a rectangular function:

where ${\displaystyle B}$ (representing its bandwidth) is an arbitrary cutoff frequency.

Its impulse response is given by the inverse Fourier transform of its frequency response:

where sinc is the normalized sinc function.