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This paper presents an introduction about the Voice over Internet Protocol (VoIP) technology and some background infor-mation about fundamental frequency of Thai language and some different characteristics of fundamental frequency of child voice, male voices and female voices. Also, voice quality indicator and an overview of voice quality measurement methods have been presented. However, only subjective measurement methods were considered in detail. Moreover, brief information about voice codecs has been described. For the approach of this study, the data set from listening-opinion tests with G.711 and G.729 has been used and analyzed. Analyzed results from T-test and ANOVA show that G.711 and G.729 provide voice quality without difference significantly, although in cases of voices from children, female and male speakers. Therefore, this paper presents the evidence which is inconsistent with the general understanding of most engineers, who implement VoIP systems.
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