Highband spectrum envelope estimation of telephone speech using hard/soft-classification
Yasheng Qian, P. Kabal · 2004
The bandwidth for telephony is generally defined to be from 300–3400 Hz. This bandwidth restriction has a no-ticeable effect on speech quality. We present an algo-rithm which recovers the missing highband parts from telephone speech. We describe an MMSE estimator us-ing hard/soft-classification to create the missing high-band spectrum envelope. The classification is motivated by acoustic phonetics: voiced vowels and consonants, and unvoiced phonemes demonstrate different character-istic spectra. The classification also captures gender dif-ferences. A hard classification on phoneme character-istic parameters, such as a voicing degree and a pitch lag, reduces the MMSE of the highband spectrum en-velope estimates. An estimator using HMM-based soft-classification can further bring down the estimated high-band spectrum distortion by taking the time evolution of the spectra into consideration. Objective measures (mean log-spectrum distortion) and spectrograms con-firm the improvement noted in informal subjective tests. 1