改进低比特率的LPC声码器

线性预测技术已广泛用于语言信号处理,特别是用于设计低比特率的声码器。但是传统的定帧长分析方法不能很好适应语言的非平稳过程,同时由此得到的语言参数（例如预测误差）用于提取音调也容易出错,因而影响了合成语言的质量。为此我们进行了三方面的改进:（1）用自适应梯型算法代替现有定帧分析的线性预测方法,以便得到更准确的声道参数;（2）有限的变帧抽样语言参数,改善了合成语音过渡区的性能;（3）改进Gold-Rabiner的基音提取技术,使音调提取方法更简单可靠。传送数据率为2400和1200比特每秒。在计算机上模拟结果表明,2.4kb/s的方案所合成的语言较为自然易懂,且不难分辨熟人口音,l.2kb/s方案的合成语言也未严重降级。采用缓存器后,两种方案均可在固定数据率信道上传输。

IMPROVEMENT OF LOW BIT RATE LPC VOCODER

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 not reliable to exltraot pitch from the parameters （e.g. residual） obtained by this method. Due to these reasons,speech quality can be adversely affected. In order to improve the quality and intelligibility,in this paper it presents improvement 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 non-uniform sampling of speech parameters with constrained interval to improve performance of synthesized speech during fast transients. （3） Improving Gold-Rabiner pitch extraction technique for saving time and reliable. 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,using transmit and receive buffers.