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.

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.>