Evaluation of In-Car Communication Systems - Digital Signal Processing for In-Vehicle Systems and Safety - page 96

Table 5.2 Weighting factors of the octave bands corresponding to [ 30 ]
Octave band k 1 2 3 4 5 6 7

W k 0.129 0.143 0.114 0.114 0.186 0.171 0.143

Center frequencies 125 Hz 250 Hz 500 Hz 1,000 Hz 2,000 Hz 4,000 Hz 8,000 Hz

where N represents the number of used modulation frequencies. Finally, the STI
can be derived by a weighted sum of the transmission indices

X 7

STI ¼ W k MTI k ; (5.13)

k¼ 1

where W k represents the octave band-weighting factor. The weighting factors are
chosen due to the psychoacoustic importance of each octave band. Octave bands,
which are essential for the hearing impression, get a larger weight than others. In this
particular case, the weights were chosen as referred in Table 5.2 corresponding to [ 30 ].
Using this approach, the single STI values were measured within different
scenarios. To measure the necessary signals, the same configuration as shown before
in Fig. 5.14 was utilized. In addition, breaks were inserted into the test signal in order
to prevent that the ICC system changes too much in its characteristics, e.g. the gain,
by detecting the test signal as noise. During the pauses, the ICC system can adjust
back to the initial settings. The test signal was transmitted via the artificial mouth
loudspeaker and again recorded the binaural ear microphones. This was carried
through different velocities and with an activated and a deactivated ICC system in
order to compare the obtained results.
Figure 5.19 depicts the results for the right ear of the listening passenger. The
measuring shows that the STI values, due to the activation of an ICC system,
are increased by about 0.15 due to the usage of an ICC system. In addition, the STI
decreases in the case of the deactivated ICC system by accelerating to higher speed.
If the ICC system is turned on, the STI values for 90 km/h and 120 km/h are nearly
the same. In this case, the ICC system compensates the decreased SNR by using a
higher amplification. The reduction of the STI from 120 to 150 km/h can be justified
by the maximum amplification of the ICC system which is obtained in between these
two velocities. Therefore, no further amplification is provided by the ICC system, and
the STI decreases due to the increased background noise level. Further evaluation
results of a similar ICC system by deriving the STI can be found in [ 27 ].

5.5.2 Quality Degradation for the Listening Passenger

The quality degradation for the listening passenger has two main reasons. The first
one is the increase of reverberation through the ICC system. The second is the
mismatch in the localization of the acoustical and visual sources of the speech