Evaluation of In-Car Communication Systems - Digital Signal Processing for In-Vehicle Systems and Safety - page 93

Fig. 5.16 Frequency response (magnitude) of the octave filters
Table 5.1 Center frequencies of the octave filters
Octave band 1 2 3 4 5 6 7
Center frequency 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz 8 kHz
To obtain a frequency-selective observation, the SNR is determined in seven
different octave bands. In Fig. 5.16 , the corresponding octave filters are depicted.
These octave bands are the same ones as used for calculating the speech transmis-
sion index as described in the next section. The test signals recorded using the
binaural microphones have to be filtered by octave filters before the SNR is
calculated according to the same approach as explained before in Eqs. 5.1 - 5.7 .
In Table 5.1 , the center frequencies of the octave filters are depicted.
To compare the results, the SNR determination was done for the activated and
the deactivated ICC system. Figure 5.17 shows the results for the right ear of the
listening passenger. A significant increase of the SNR, especially in the octave
bands 5-7, is observed.
At this point, only the SNR improvement achieved at the ears of the listening
passenger is determined. Another interesting issue is the increase of the noise power
inside the passenger compartment caused by the ICC system. The desirable situa-
tion in this case would be to avoid an increase in the noise power. Therefore, a
measurement of the noise power difference between an activated and a deactivated
ICC system measured at the listening passenger's ears gives also an indication
about the quality of the ICC system.