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The non-linear nature of low-rate parametric speech coding has rendered questionable analytical methods for estimating the end-to-end voice quality of interconnected telecommunications networks. Instead, quantification of transmission performance appears to require direct subjective evaluation of the pertinent conditions of interest. At the same time, the rapid growth of cellular communications has highlighted the need to characterize the quality of switched networks when cellular terminals are attached at the networks' termination nodes. In this paper the voice quality of interconnected NorthAmerican and European digital cellular systems over national and international transmission facilities is quantified. From these assessments it was concluded that cellular networks employing the TIA IS-54 8 kbit/s VSELP or the GSM 13 kbit/s RPE-LTP algorithms may meet the end-to-end quantization distortion allocation criteria when interconnected with the switched network.

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It has often been suggested that digital voice coder performance may depend on the type of listening set used in listener opinion assessments. In this letter, the quantitative impact of using headphones and telephone handsets is presented for single-stimulus MOS assessments. From this analysis, it is derived that there are significant numerical differences between tests employing these types of listening instruments and that cardinal differences in performance may be consistent and thus predictable. This study is extended to an ordinal analysis from which it is derived that although ranked performance does vary, depending on which of two listening instruments is employed, the two rankings are highly correlated.