Study On Artificial Bandwidth Extension Of Speech

Nowadays,in most of telecommunication systems,the bandwidth of speech signal is limited to 4kHz.In this case,the limited frequency range leads to restricted speech quality compared to wideband speech. While wideband speech coding technology improves the speech intelligibility and naturalness, telecommunication systems have to be modified. However,it requires a long transitional period to change the telecommunication systems due to economical and historical reasons. One possible solution is to artificially extend the signal bandwidth at the receiving end by reconstructing the wideband speech. This process of regenerating wideband spectra from narrowband signals is referred to as artificial bandwidth extension.Bandwidth extension can be divided into training and extending procedures.The purpose of training procedure is to find a proper mapping function, while the purpose of extending part is to transform the parameters and synthesis the speech.In this thesis, a bandwidth extension algorithm based on codebook mapping is introduced.This algorithm consists of three main modules:the feature extraction,the extension of spectrum and the extension of excitation.The main tasks in the thesis are as follows:(1) The three submodel of digital model of voice singal:excition model,acoustic tube model and radiation are studied.The pre-processing and linear prediction of speech are also introduced.(2) The pitch period estimatoin,LPC-derived cepstral coefficient,Mel-cepstral coefficient are described.(3) The principle of vector quantization(VQ) and the measure of VQ distortion are studied; the best algorithm to build a codebook is introduced and LBG algorithm is adopted to designing the codebook.(4) The bandwidth extension algorithms,including codebook mapping,linear mapping and statistic mapping,are studied. A new bandwidth extension algorithm based on codebook mapping is proposed.(5) The performaces of the propoed algorithm are evaluated by experiments and the results of objective and subjective tests are given.