We can write the frequency response of a filter in terms of real and imaginary part as
where HR(ω) and HI (ω) are the real and imaginary part of H(ω).
We can also write H(ω) in terms of magnitude response |H(ω)| and phase response θ (ω) as
where H(ω) is a continuous function of ω. If the frequency response of a digital filter is expressed in the following continuous phase form
we can say that it is in linear phase form τp