H. Najaf-Zadeh and P. Kabal
"Narrowband Perceptual Audio Coding: Enhancements for Speech", Proc. European Conf. Speech Commun. Technol. (Aalborg, Denmark), pp. 1993-1996, Sept. 2001.
This paper presents a bi-modal coding paradigm to compress narrowband audio signals at 8 kbit/s. In the general mode, the Enhanced Narrowband Audio Coder (ENPAC) exploits the characteristics of the human hearing system to adaptively code the perceptually important spectral components of the input audio. The other mode is employed to handle audio inputs with a strong harmonic structure. In that mode, the input block is represented by its audible harmonics. The spectral magnitude is modeled by the linear prediction analysis in the time domain. The phase of each harmonic is predicted and the phase residues are quantized using an adaptive bit allocation algorithm. This paper introduces a perceptually-based upper bound for phase errors of spectral components. The ENPAC encoder delivers good quality for narrowband speech and non-speech inputs.
P. Kabal and W. B. Kleijn
"All-Pole Modelling of Mixed Excitation Signals", Proc. IEEE Int. Conf. Acoustics, Speech, Signal Processing (Salt Lake City, UT), pp. 97-100, May 2001
Conventional Linear Prediction (LP) techniques can fail to adequately model speech spectra when the model order is too low and/or when the input is periodic (voiced speech). In this paper, we view the LP modelling problem as a correlation matching problem. We introduce a correlation matching criterion which models the signal as altered mixture of a noise-like excitation and a periodic excitation. As such it is an extension of the Discrete All-Pole (DAP) modelling approach. The new technique provides a means to generate LP spectra that evolve more smoothly from frame to frame even when the excitation signal has a periodic component with changing period.