What are linear phase filters and why are they important?

Linear phase filters have phase response that is linear function of frequency: phi(w) = -alpha*w, causing constant group delay. All frequency components are delayed equally, preserving waveform shape. FIR filters achieve linear phase with symmetric (Type I, II) or antisymmetric (Type III, IV) coefficients. IIR filters cannot have exactly linear phase. Importance: Critical in audio (phase distortion audible), communications (ISI in data transmission), imaging (edge preservation), and measurement systems. When phase distortion is unacceptable, use linear phase FIR despite higher order requirement.