IP Phone Listening Speech Quality

IP Phone Listening Speech Quality

The same precautions and limitations as listed in chapter 4, page 22 for listening speech quality analysis and result interpretation for gateways need to be considered for IP phones:

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  TOSQA2001 is limited to one-way transmission scenarios for IP phones. Conversational aspects like double talk, echo disturbances or background noise are not considered.
  
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  The TMOS results do not cover the overall quality of an IP phone under test.
  
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  The algorithm uses real speech as stimulus. The results therefore directly depend on the choice of speech sample. Exchanging the speech sample, e.g. from German speech to English speech, will lead to a different analysis result. Even the use of different sentences of the same language will lead to –slightly– different results. In continuation of the previous SQTE’s, German speech samples were used.
  
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  TOSQA2001 can not provide 100% accuracy. The accuracy can never exceed the accuracy of a subjective test.
  
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  TMOS differences of 0.1 or 0.2 are not regarded as significant in the analysis. Differences of 0.2 show a tendency and differences of 0.3 are seen as relevant.
  

The IP impairments are introduced by NISTnet. In network jitter tests NISTnet introduces an additional delay. During the TMOS analyses the measured delay values -representing the end-to-end delay- are corrected by the NISTnet delay.

3.8G.711

The graphical representation of these results is shown in figure 6.1. The three different IP phones are represented by the different colors. The dashed lines indicate the maximum and minimum scores, the average is given by the solid black line. The results can be summarized as follows:

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  The quality differences between the three devices are very high.
  
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  Listening speech quality in sending direction differs significantly by 0.6 TMOS.
  
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  Two phones provide a similar performance in receiving direction for the different application forces of 2 N, 8 N and 13 N. The third phone (blue curve) provides a significantly better performance. The TMOS differences can be determined to approximately 1.5 points.
  
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  The hands-free mode was only tested for two devices. The quality differs significantly for both phones depending on the transmission direction.
  
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  The delay is nearly constant for the three devices under test.
  

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  Listening speech quality slightly degrades for the three IP phones under the influence of packet loss.
  
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  Jitter is completely covered by the implementation represented by the blue curve.
  
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  The delay is in a comparable range for the three implementations without IP jitter. The different jitter buffer lengths lead to delay differences of approximately 40 ms.