A Novel Method to Improve Pitch and Voicing Strength Estimation for Low Bit Rate Speech Coding

In low bit rate speech coders, pitch is usually transmitted once per frame and, when needed, the intermediate pitch values are obtained by interpolation between two adjacent pitch values. Although pitch usually evolves slowly, sometimes it has non-linear variation and the estimated pitch differs from the real one. In addition, some speech coders, e.g. waveform interpolation coder, rely on smooth pitch-cycle evolution to extract speech model parameters in analysis stage. However, non-stationary characteristics of speech may lead to inaccurate estimation of the parameters. This affects the synthesised speech quality. We propose to use a pre-processor, which modifies the residual speech signal to provide smooth pitch period and pitch-cycle evolutions without distorting perceptual speech quality. Thus, the pitch and the voicing level can be determined correctly