| The VoIP technology belongs to the category of voice communication on packet switched networks. The earliest study started at the beginning of 70's last century, and the focus has been turned from theoretical research to practical application with the rapid development of Internet. The IP Phone using VoIP technology is the result of the combination of such two chief technologies, the packet technique on network taking IP as its sign, and the integrating technique of services on network taking multimedia as its goal. It has become the joint of traditional telecom and IP network, mutually supplying with each other's advantages.Up to now VoIP has implemented a majority part of traditional PSTN functions, based on which people raised additional requirement on speech quality that is not satisfied in VoIP now。The major factor to improve speech quality is to decrease the occupation of the network resources .voice activity detection(VAD) can cull the unless speech according to characteristics of people's speech to economize the network resources. Under such a background, as a part of the research work on VoIP gateway, the author carried through some deep study into VAD.. By analyzing and comparing the VAD algorithm that is lately prevalent , I demonstrate a fusion VAD algorithm which consists of two parts—adaptive energy detection(AED) and linear prediction model detection(LPMD) .AED tests the noise by comparing the energy of the signal, while LPMD tests the noise by analyzing the difference between noise and speech in the signal model.. This fusion algorithm have the advantages of two algorithms and greatly advances in the capability of testing.The article first introduces some background knowledge about IP phone, necessary technologies and possible solutions. The importance of study about VAD is described. Based on some latest VAD technologies, aiming at DSP implementation, an improvement in theory and optimization on practical is given on every main part of VAD.
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