Evaluation of In-Car Communication Systems - Digital Signal Processing for In-Vehicle Systems and Safety - page 75

increasing and nowadays clearly higher than during the development of the first
hands-free systems. This allows higher sampling rates and thus a larger signal
bandwidth, which is essential for natural-sounding speech output. But still
limitations in the algorithmic complexity are given and lead to additional boundary
conditions that need to be considered during the design of an ICC system. In
addition, limitations due to the characteristic of the electroacoustic transducers
such as the bandwidths and the positions of the utilized microphones have to be
considered for the design of ICC systems.
5.2.2 Listening Passenger
The main objective of ICC systems is to increase the speech intelligibility for the
listening passenger. This is done by amplifying the speech signal of the talking
passenger and thereby increasing the SNR at the ear of the listening passenger. This
amplification cannot be increased to an arbitrarily high gain, due to the already
mentioned physical limits. But how much improvement in terms of amplification is
actually desired by the listening passenger? In most cars, the communication from
the rear to the front passengers is marginally impaired, even at medium or high
velocities. The usage of an ICC system would make the car unreasonably more
reverberant and thus decrease the communication quality from front to rear. How-
ever, the opposite direction (from rear to front) behaves differently. The communi-
cation quality is impaired much more, especially at higher velocities.
The reason for the quality difference between both communication directions
originates from the directionality of the human mouth. Figure 5.6 shows the average
directionality for two frequency ranges when the driver is speaking [ 8 ]. In addition,
the sound propagation to the other passengers is shown schematically. 5 For
frequencies between 1,400 and 2,000 Hz, an attenuation of more than 10 dB
between the front side (at 0 ) and the back of the head (at 180 ) can be measured.
Due to this directionality and the arrangement within the passenger compartment,
the rear passengers (especially the one directly behind the talking passenger) have
problems to understand the front passengers.
Another effect that influences the boundary conditions of the amplification is the
background noise level. As alreadymentioned above, the background noise consists of
different sources, for example, engine noise, wind noise, and tire noise. Due to the fact
that at standstill or low velocities the noise level is very low, the SNR is quite high, and
therefore, there is no need to use an ICC system. 6 But if the car accelerates to higher
velocities, the background noise level increases by more than 30 dB at all relevant
frequencies, and the usage of ICC systems can improve communication quality.
5 Effects due to the boundaries of the passenger compartment are not mentioned here.
6 This may not be true for vans with more than two seat rows or even for buses.