HYBRID HARMONIC CODING OF SPEECH AT LOW BIT-RATES

This paper presents a novel approach to sinusoidal coding of speech which avoids the use of a voicing detector. The proposed model represents the speech signal as a sum of sinusoids and bandpass random signals and it is denoted hybrid harmonic model in this paper. The use of two different sets of basis functions increases the robustness of the model since there is no need to switch between techniques tailored to particular classes of sounds. Sinusoidal basis functions with harmonically related frequencies allow an accurate representation of the quasi-periodic structure of voiced speech but show difficulties in representing unvoiced sounds. On the other hand, the bandpass random functions are well suited for high quality representation of unvoiced speech sounds, since their bandwidth is larger than the bandwidth of sinusoids. The amplitudes of both sets of basis functions are simultaneously estimated by a least squares algorithm and the output speech signal is synthesized in the time domain by the superposition of all basis functions multiplied by their amplitudes. Experimental tests confirm an improved performance of the hybrid model for operation with noise-corrupted input speech, relative to classic sinusoidal models which exhibit a strong dependency on voicing decision. Finally, the implementation and test of a fully quantized hybrid coder at 4.8 kbit/s is described.