Phase equalization using a hybrid cascade of causal and noncausal IIR allpass filters

Abstract

The perceptibility of audio signal phase and the need for coordination among multiple loudspeakers require the design of infinite impulse response (IIR) allpass filters for phase equalization. This paper proposes a generalized method for automatically designing IIR allpass filters using a hybrid cascade of causal and noncausal low-order IIR allpass filters. Cascading noncausal allpass filters can produce negative group delay, providing additional flexibility in shaping the group delay of the IIR allpass filter. The proposed method offers more relaxed constraints, making it better suited for designing IIR allpass filters with complex phase responses. Two IIR allpass filters are designed to equalize the phase responses of the woofer and tweeter, and their performance is compared with two existing design methods. The design results demonstrate that the proposed design method effectively equalizes the phase, minimizes filter order, and reduces computational cost.