Amman, S., Charette, F., Nicastri, P., Huber, J. et al., "Quantifying Hands-Free Call Quality in an Automobile," SAE Int. J. Passeng. Cars - Mech. Syst. 8(3):1104-1109, 2015, doi:10.4271/2015-01-2335.
Hands-free phone use is the most utilized use case for vehicles equipped with infotainment systems with external microphones that support connection to phones and implement speech recognition. Critically then, achieving hands-free phone call quality in a vehicle is problematic due to the extremely noisy nature of the vehicle environment. Noise generated by wind, mechanical and structural, tire to road, passengers, engine/exhaust, HVAC air pressure and flow are all significant contributors and sources of noise. Other factors influencing the quality of the phone call include microphone placement, cabin acoustics, seat position of the talker, noise reduction of the hands-free system, etc. This paper describes the work done to develop procedures and metrics to quantify the effects that influence the hands-free phone call quality. It will be shown that a listening study of using 49 evaluators, indicated that the ETSI EG 202 396-3EG (VoIP Standard) for SMOS (Speech Mean Opinion Score) and NMOS (Noise Mean Opinion Score) correlates better than the ETSI TS 103 106 (Mobile Standard) for speech and noise ratings when quantifying the quality of a hands-free phone call. However, ETSI TS 103 106 was found to correlate better for GMOS (Global Mean Opinion Score). Using these results, MOS scores can be calculated to investigate the influences of factors that can influence the quality of a hands-free call. Two examples are given in this paper. The first example investigates the effect of background noise level in the vehicle cabin on hands-free phone call quality for two suppliers of such systems. The second example explores the effect of noise suppression level, as set by the hands-free system, for two suppliers. The relationships of these effects to SMOS and NMOS are developed and discussed.