Evaluation of In-Car Communication Systems - Digital Signal Processing for In-Vehicle Systems and Safety - page 78

quality, potentially remaining noise components are amplified and played back over
the loudspeakers. This may lead to an undesirable SNR reduction. In the worst case,
the residual noise components are fluctuating, and the listening passengers are
disturbed by the noise level changes while the ICC system is activated. As a result,
the gain of the ICC system and the maximum attenuation of the noise suppression
have to be adjusted such that neither a noise level increase nor noise level
fluctuations due to the ICC system can be perceived by the listening passengers.
5.2.3 Talking Passenger
The talking passenger has one main requirement for the ICC system: He simply
does not want to be aware of the system. 10 The ideal case for the talking passenger
would be if the situation within the compartment is the same whether with or
without an ICC system. But since the speech signal is amplified by the ICC system
and reproduced via the loudspeakers, it may occur that he hears himself while
speaking. This effect is highly correlated with the gain and the delay of the ICC
system. If the delay is too high, the talking passenger hears his own speech even at
low gain levels as an echo.
Finally, it should be mentioned that ICC systems increase the degree of reverber-
ation. This increase also depends on the delay and the gain of the system. The larger
both are, the more the reverberation increases. This effect is tolerated up to a certain
degree if the communication quality increases at the same time. However, if the
reverberation time is increased up to more than 80-140 ms (depending on the type of
car), passengers start complaining about the reverberant character of the vehicle [ 7 ].
5.3 Signal Processing for In-Car Communication Systems
As mentioned during the introduction of this chapter, some basic knowledge of the
involved signal processing units of ICC systems is necessary if appropriate evalua-
tion procedures should be investigated. For that reason, we will present brief
descriptions of the individual signal processing blocks shown in Fig. 5.5 in this
section before starting with details about the evaluation in the upcoming sections.
10 This statement can be slightly modified for the listening passengers: The listening passengers
should not be aware of a proper operating ICC system (in terms of localization, reverberation,
noise increment, etc.) unless the system is switched-off.