A case study of perceived listening quality of temporally interrupted VoIP service

In modern VoIP services, we often observe temporary speech interruptions during ordinary conversations. This is caused by the mobility facility and the best effort nature of data transport networks. This impairment factor has been classically de-emphasized by existing subjective and objective quality assessment techniques because it is rarely observed over legacy landline telephone systems. This paper explores the perceptual effects of discontinuity in speech communications. A series of lab-based subjective tests has been carried-out in order to understand the perceptual quality variation with respect to diverse patterns of temporal service discontinuity. In parallel, impairment conditions have been evaluated using the standardized active and passive signal-layer SQA (Speech Quality Assessment) models described in ITU-T Rec. P.862 and P.563, respectively. Our exploration indicates that both strategies estimate poorly perceived quality of interrupted speech stimulus on a sample-by-sample basis. We found that the time-alignment algorithm of original and degraded speech sequences embedded in the ITU-T Rec. P.862 SQA model plays an essential role in the observed unpredictable quality rating estimates. Moreover, the dichotomy treatment of discontinuity instances by the ITU-T Rec. P.563 SQA model constitutes a principal source of inaccuracy of estimated perceptual quality. A guideline for proper consideration of discontinuity distribution and context is presented and applied on the ITU-T Rec. P.862 SQA algorithm. This results in an improvement of its estimation performance in the context of interrupted speech sequences.