Postfiltering Techniques in Low Bit - Rate Speech

Postfilters are used in speech decoders to improve speech quality by preserving formant information and reducing noise in the valley regions. In this thesis, a new adaptive least-squares LPC-based time-domain postfilter is presented to overcome problems presented in the conventional LPC-based time-domain postfilter. Conventional LPC-based time-domain postfilter [4] produces an unpredictable spectral tilt that is hard to control by the modified LPC synthesis, inverse, and high pass filtering, causing unnecessary attenuation or amplification of some frequency components that introduces muffling in speech quality. This effect increases when voice coders are tandemed together. However, the least-squares postfilter solves these problems by eliminating the problem of spectral tilt in the conventional time-domain postfilter. The least-squares postfilter has a flat frequency response at formant peaks of the speech spectrum. Instead of looking at the modified LPC synthesis, inverse, and high pass filtering as in the conventional time-domain technique, a formant and null simultaneous tracking technique is adopted by taking advantage of a strong correlation between formants and poles in the LPC envelope. The least-squares postfilter has been used in the 4 kb/s Harmonic Excitation Linear Predictive Coder (HE-LPC) and subjective listening tests indicate that the new postfiltering technique outperforms the conventional one in both one and two tandem connections. Thesis Supervisor: Dr. Suat Yeldener Title: Scientist, Voiceband Processing Department, Comsat Laboratories Thesis Supervisor: Dr. Thomas F. Quatieri Title: Senior Member of the Technical Staff, MIT Lincoln Laboratory