The quality delivered by low-rate parametric speech coding deployed in commercial mobile satellite systems has to be assessed by formal subjective assessment methods due to their non-linear nature. Aeronautical systems, especially those for deployment in small aircraft, pose yet more stringent conditions due to high environmental noise levels. In this paper the results of an evaluation of the voice quality of DVSI's 4.8 kbit/s Advanced Multi-Band Excited (AMBE) coding under aeronautical channel conditions are presented for listening and conversational experiments. From these assessments it was concluded that that the voice quality of the Inmarsat Aeronautical system will be perceptibly improved when the current full-rate codec is replaced by the AMBE codec.

The non-linear nature of low-rate parametric speech coding has made it necessary to resort to formal subjective assessments for quantifying end-to-end voice quality of interconnected networks. At the same time, the rapid growth of cellular communications has highlighted the need to characterize transmission quality when cellular terminals are attached at the access or termination nodes of switched networks. In the paper the voice quality of interconnected North-American and Japanese digital cellular systems over public transmission facilities is quantified. From these assessments it is concluded that cellular networks using 8 kbit/s or 6.4 kbit/s VSELP may meet end-to-end quantization distortion criteria when interconnected with the switched network.

This paper provides information on a laboratory study conducted by COMSAT Laboratories to evaluate the effect of echo and delay on facsimile call retention rates. The term "call retention" is defined as the ability of a pair of facsimile terminals to maintain a circuit connection until all pages have been successfully transmitted after negotiation of the high speed signaling rate. The results of this study indicate that the effect of network echo has the potential of degrading facsimile call retention. In addition, propagation delay reduces retention rates even further. The results of this work are currently under consideration by the International Telecommunication Union (formerly CCITT) in the formulation of test methodologies for assessing facsimile terminal implementation performance.

The design and analysis of an experiment, which was conducted in order to assess the subjective impact of transmission delay on mobile voice communications via a satellite, are presented. This study was conducted using simulated end-to-end propagation delays representative of low Earth orbit (LEO) and geostationary Earth orbit (GEO) networks (single and double-hop) in combination with three different speech coding technologies: 64 kb/s pulse code modulation, 8 kb/s vector-sum excited linear prediction, and 4.15 kb/s improved multiband excitation (representing wireline, cellular, and mobile satellite technologies, respectively). Subsequent analysis included computation of the cross-correlation between the delay and coding method, from which it was concluded that for "wireline-quality" and "communications-quality" circuits the transmission performance somewhat degraded as the propagation delay increased to that of a double-hop GEO circuit. However, for "communications quality" circuits employing the 4.15 kb/s speech coding algorithm, the transmission performance remained constant as propagation delay increased.>

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.>

The effect of using headphone versus telephone handsets is characterized for degradation mean opinion score (DMOS) assessments. From this, it is derived that differences in opinion scores are dependent on the type of listening instrument employed. Further, in the middle of the quality range, opinion scores derived from tests employing telephone handsets are higher than those derived from tests employing headphones.>

The standardization of high-quality speech coding has intensified. In parallel, a number of novel applications are placing new demands on transmission efficiency and quality. In response to such challenges, standardization bodies have begun the definition of requirements for the next generation of very low-rate speech coding. Taking a lead in these activities, ANSI committee T1A1 and the ITU-T initiated the definition of the performance and characteristics of a wireline-quality 4-kb/s speech coding algorithm for network applications. This emerging set of requirements is presented.>

An analytic expression is derived that characterizes the distortion occurring in an adaptive differential pulse code modulation (ADPCM) system, when the system encoder and decoder mistrack, in the presence of quantization and coded channel noise. These expressions demonstrate that, when such mistracking occurs, the presence of channel and quantization noise is further accentuated by the transfer function of the mistracked coder system. These results are subsequently extended to the case of coarse quantization where it is shown that in addition to the previous disorder a further term is introduced into the coder transfer function that, depending on the coarseness of the encoder quantization process, can derive the compound system unstable even when the decoder is tracking the encoder.>