改进的MELP低速率语音编码器

Improved MELP speech coder for low bit rate speech coding

在混合激励线性预测(MELP)语音编码算法中,语音帧的突变转换导致合成语音质量的下降成为一个突出问题。为解决该问题,提出一种基于过渡帧判决算法的改进MELP模型,提高了参数估计的准确度,有效实现了语音的降噪处理。引入动态清浊音判决(U/V判决)门限将语音帧分为3种类型:浊音帧、清音帧、过渡帧,通过区分过渡帧和清浊音帧,避免了传统的U/V判决错误和清浊音帧的突变转换。给出了改进MELP编码参数比特分配表,通过PESQ—MOS测试表明,合成语音质量尤其是高频女声合成语音质量有了明显的改进。

In the Mixed Excitation Linear Prediction（MELP） algorithm, frames with excessive switchings result in quality degradation for synthesized speech,which is a serious problem.In order to solve this problem,a new approach is given to analyze and synthesize the transition segments which are called the transition frames.This can improve the accuracy of parameter estimation and reduce the noise caused by classifying the transition frame simply into U/V frame.A new frame type decision algorithm with dynamic U/V threshold related to the pitch lag is introduced which classifies the speech frame into three types：voiced, unvoiced,and transition.The classifier can reduce the U/V decision errors and avoid excessive switchings between voiced frame and unvoiced frame.An improved bit allocation table is introduced and the PESQ--MOS test shows that the synthesized speech quality has been greatly improved through the new MELP speech coder,especially for high frequency female speakers.