An adaptive post-filtering technique based on the modified Yule-Walker filter

This paper presents an adaptive time-domain post-filtering technique based on the modified Yule-Walker filter. Conventionally, post-filtering is derived from an original LPC spectrum [1]. In general, this time-domain technique produces unpredictable spectral tilt that is hard to control by the modified LPC synthesis, inverse and high pass filtering and causes unnecessary attenuation or amplification of some frequency components that introduces muffling in speech quality. This effect increases when voice coders are tandemed together. Another approach of designing a post-filter was developed by McAulay and Quatieri [2] which can only be used in sinusoidal based speech coders. We have also developed another new time-domain post-filtering technique. This technique eliminates the problem of spectral tilt in speech spectrum that can be applied to various speech coders. The new post-filter has a flat frequency response at the formant peaks of speech spectrum. Instead of looking at the modified LPC synthesis, inverse, and high pass filtering in the conventional time-domain technique, we gather information about the poles of the LPC spectrum in the new technique. This post-filtering technique has been used in a 4 kb/s Harmonic Excitation Linear Predictive Coder (HE-LPC) and a subjective listening tests have indicated that this technique outperforms the conventional one in both one and two tandem connections.