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.
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机译：提出了一种在LPC模型中激励信号的两频带逼近的新方法，以提高极低速率编码中的语音自然性。该方法基于简化的多频带激励模型，利用频域中的瞬时频率（IF）估计概念准确确定了周期性程度。基于中频的平坦度，确定单个频带内LPC残留频谱中的谐波结构，作为音高和发声检测的标准。在此基础上，通过组合较低频段中的预定义周期信号和较高频段中的随机信号来对激励进行建模。结果表明，与使用传统的二进制激励所获得的语音相比，这在非常低的速率编码中大大改善了重建语音的自然性。还在基于时间分解（TD）的800 b / s编码中给出了该技术的性能。
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