A new method for two-band approximation of excitation signals in an LPC model, to improve speech naturalness in very low rate coding, is proposed. Based on a simplified model of multi-band excitation, the method accurately determines the degree of periodicity, using the concept of instantaneous frequency (IF) estimation in the frequency domain. The harmonic structure in the spectrum of LPC residual, within individual bands, is identified based on flatness of the IF as a criterion for pitch and voicing detection. On this basis, the excitation is modelled by combining a predefined periodic signal in the lower band and a random signal in the higher band. It is shown that this improves considerably the naturalness of reconstructed speech in very low rate coding in comparison with that obtained using traditional binary excitation. The performance of the technique is also given in temporal decomposition (TD) based coding at 800 b/s.
['Shahrokh Ghaemmaghami', 'Mohamed Deriche']
A new approach to modeling excitation in very low-rate speech coding