IMPROVEMENT OF LOW BIT RATE LPC VOCODER
J Wang · 1986
Linear predictive technique has been widely used for speech signal processing, especially for low bit rate vocoder in recent years. However the conventional fixed frame processing does not suit well with speech signal for its nonstationary behavior. Also it is unreliable to extract pitch from the parameters (e.g. residual) obtained by that method. Due to these reasons, speech quality may be adversely affected. In order to increase the quality and intelligibility, this paper presents improvements to the following three aspects: (1) Using adaptive ladder form algorithm instead of conventional LPC to obtain more accurate estimation of the vocal-tract transfer function. (2) Adapting nonuniform sampling of speech parameters with constrained interval to improve performance of synthesized speech during fast transients.(3) Improving Gold-Rabiner pitch extraction technique for simplicity and reliability. The computer-simulated results show that the 2.4 kb/s vocoder gives the synthesized speech with good intelligibility and quality, and the quality of the 1.2 kb/s one is not degraded seriously. Transmission over fixed rate channels can be accomplished by using transmitting and receiving buffers.