A new VBR WI coder with CW modification for embedded applications

Abstract

This dissertation presents a modified characteristic waveform (CW) decomposition method to enhance the speech quality of the wideband waveform interpolation (WI) coder and a new variable bit rate (VBR) coding technique to reduce the bit rate of the WI coder. In the original WI scheme, a CW is decomposed into a slowly evolving waveform (SEW) and a rapidly evolving waveform (REW) in Cartesian coordinates. This may deteriorate the spectral shape of the reconstructed CWs. Especially, speech quality degradation is inevitable when the REW contains SEW components. To solve this problem, the decomposition of a CW is performed on polar coordinates so that the spectral envelop information of the reconstructed CWs holds consistency even for the REW with voiced components. The proposed CW modification results in the PESQ (Perceptual Evaluation of Speech Quality) score improvement by 0.3 from the baseline speech quality, i.e., the PESQ score of 2.8. The variable bit rate scheme in the WI coder was already proposed by Plante. It takes the benefit of time varying property of speech signals. On the other hand, the target of our variable bit rate scheme is to utilize the slowly varying property of the signal. After the WI coder extracted all parameters, the distortions between the current and the predicted parameters are measured. The predicted parameters are acquired by the prediction based on the past parameters to be transmitted. A parameter would not be transmitted unless the distortion exceeds the preset threshold. At the decoder, the non-transmitted parameter is reconstructed by the same prediction method used for the encoder. In this way, we can reduce 41 percent of the total bit rate while retaining the speech quality degradation below 0.1 PESQ score. Recently, demands for the speech coders which can provide good speech quality even at very low bit rates are increasing. Especially, adequate speech coders for the embedded applications are highly demanded. The final ve...