Wideband Hands-Free in Cars – New Challenges for System Design and Testing - Digital Signal Processing for In-Vehicle Systems and Safety - page 111

Fig. 6.10 Filter characteristics (subset of test conditions)

A total number of 33 test conditions, including the reference scenarios (infinite
echo attenuation) and different combinations of delay, echo attenuation, and spec-
tral shaping, were included:

￿ Round-trip delays between 100 and 500 ms
￿ Echo attenuation between 35 and 55 dB
￿ Simulation of nonlinear residual echoes

The spectral echo content was realized by the following filter characteristics
(subset of test conditions):

￿ NB: narrowband filter, 300-3.4 kHz
￿ HF1: 3.1-5.6 kHz
￿ HF2: 5.2-8 Hz
￿ 1/3 oct.no 1: 900-1,120 Hz
￿ 1/3 oct.no 5: 2.24-2.8 kHz
￿ 1/3 oct.no 7: 3.55-4.5 kHz
￿ 1/3 oct.no 8: 4.5-5.6 kHz

The 1/3 octave filter characteristics are shown in Fig. 6.10 together with the
hearing and speech perception threshold. These filters seek a more detailed analysis
of the critical frequency range between 1 and 5 kHz which provides the highest
sensitivity for sound and speech perception.
A 5-point annoyance scale was used (5 points: Echo is inaudible, , 1 point:

Echo is very annoying [ 18 ]). The stimuli were presented without pair comparison.
The results were analyzed on a MOS basis together with confidence intervals based

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