Characteristic waveform (CW) decomposition based on discrete cosine transform (DCT) is proposed firstly in waveform interpolation (WI) speech coding.The complicated CW alignment operation is cancelled by using this method.Secondly

a new unvoiced/voiced phase decision method and the classification method of voiced phase are proposed with respect to phase reconstruction.Finally

DCT-WI coders at 2.0kbps and 1.6kbps are designed

respectively.The subjective MOS test results indicate that the 2.0kbps DCT-WI coder outperforms 2.4kbps MELP coder and the 1.6kbps DCT-WI coder also has a good performance.