低延迟话音编码算法的增益优化与混合窗设计

G.728标准将增益滤波器的偏移不适当地处理成常数32dB,相当于将20～30ms内短时平稳的语音信号不适当地按整个时间轴上的平稳信号对待。对增益滤波器的改进必然涉及优化混合窗。该文提出一种估计平均分段信噪比的方法用来设计混合窗参数,并导出激励增益的精确表示。以此为基础,将Jayant的自适应量化与G.728的增益预测结合起来,提出一种适合于设计低时延码激励线性预测(LD-CELP)的增益码书的方案。用此方法,激励的精确值经自适应预测和量化后,3bit自适应量化和4bit自适应量化分别比G.728的固定量化有0.5dB和6dB的信噪比的改善。当采用4bit自适应量化和64码字的波形码书,比G.728复杂性降低20%,同时信噪比提高1dB。

Optimizing Gain and Designing Hybrid Window of LD-CELP

The offset of gain filter is dealt unsuitably with a constant 32dB in G728 standard. That is, the short-term stationary speech signal during the duration 20-30ms is unsuitably supposed as stationary at whole time axis. It is necessary to design optimizing hybrid window for improving gain filter. In this paper, the exact expression of excitation gain is derived and an estimation method of SNRav used to optimize the parameters of hybrid window is presented. On the basis of above, this paper combines the Jayant adaptive quantization and G728's gain prediction to propose a scheme fit for designing gain codebook of Low Delay Code Excited Linear Prediction(LD-CELP), in which the exact gain value is adaptively predicted and quantized. In this method, for adaptive quantization gain of 3 bit and 4 bit, the SNR increases by 0.5 and 6 dB separately than that of G.728. If adopting 4 bit adaptive quantization and the shape codebook of 64 codevectors, the G728' complexity can reduce by 20% while SNR has 1 dB improvement.