A linear-phase filter is typically used when a causal filter is needed to modify a signal's magnitude-spectrum while preserving the signal's time-domain waveform as much as possible. Linear-phase filters have a symmetric impulse response, e.g.,
The symmetric-impulse-response constraint means that linear-phase filters must be FIR filters, because a causal recursive filter cannot have a symmetric impulse response.
We will show that every real symmetric impulse response corresponds to a real frequency response times a linear phase term , where is the slope of the linear phase. Linear phase is often ideal because a filter phase of the form corresponds to phase delay
and group delay
That is, both the phase and group delay of a linear-phase filter are equal to samples of plain delay at every frequency. Since a length FIR filter implements samples of delay, the value is exactly half the total filter delay. Delaying all frequency components by the same amount preserves the waveshape as much as possible for a given amplitude response.