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Tham khảo tài liệu 'voip technologies part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Assessment of Speech Quality in VoIP 41 Fig. 9. Objective assessment of an impact of phonetic elements by PESQ method. The average MOS score of subjective listening test and objective speech quality assessment by PESQ is summarized in Table 6. The average subjective score is always higher than the results of objective evaluation. The difference is negligible for vowels diphthongs however it is more significant in case of nasals liquids and plosives affricates. Group Subjective score MOS Objective PESQ score MOS Vowels diphthongs 2.81 2.70 Nasal liquids 2.96 2.52 Plosives affricates 3.85 3.42 Fricatives 3.36 3.31 Table 6. Average MOS score of each group of phonetic elements. The difference in speech quality of groups containing only voice sounds and groups containing also unvoiced sound is considerable in results of both subjective as well as objective tests. For example the speech quality is roughly by 1 MOS higher if the packet losses hit only plosives and affricates than if the losses are in vowels and diphthongs. This fact should influence the design of packet loss concealment mechanisms to put more focus on elimination of losses of vowels diphthongs nasals or liquids. 6. Conclusions This chapter provides an overview on the speech quality assessment in VoIP networks. Several effects that can influence the speech quality are investigated by objective PESQ and or subjective tests. The results of objective tests show advantage of wideband communication channel only for high quality networks with PLR up to 4 . On the other hand while the speech is affected by consecutive packet losses or by individual losses with higher packet loss ratio the narrowband channel reaches better score. The most significant difference between wide and narrow band speeches is at 12 of lost packets. The consecutive packet losses can leads to the higher speech quality while the duration of losses is long enough comparing to the individual losses. The exact duration of loss that reaches .
