In-Car Communication (ICC) systems strive to enhance communication among passengers within a vehicle by compensating for acoustic loss between two dialog partners. There are several reasons for such an acoustic loss. For example, typically, the driver cannot turn around to listeners sitting on the rear seats of the vehicle, and therefore he speaks towards the wind shield. This may result in 10-15 dB attenuation of his speech signal.
To improve the intelligibility and sound quality in the communication path from front passengers to rear passengers, the speech signal is recorded by one or several microphones, processed by the ICC system and played back at the rear loudspeakers. Bidirectional ICC systems enhancing also the speech signals of rear passengers for front passengers may be realized by using two unidirectional ICC instances.
FIG. 1 shows an exemplary system for two acoustic zones which are represented by driver/front passenger and rear passengers. The signal processing modules used in each of the two zones of such a system usually include beamforming (BF), noise reduction (NR), signal mixing (e.g. for driver and front passenger), Automatic Gain Control (AGC), feedback suppression (notch), Noise Dependent Gain Control (NDGC) and equalization (EQ) as shown in FIG. 2. Beamforming steers the beam of a microphone array to dedicated speaker locations such as the driver's or co-driver's seat. Noise reduction is employed to avoid or at least to moderate background noise transmitted over the ICC system. In addition, sibilant sounds may be reduced by a so-called deesser. Since speakers generally differ in their speaking habits, especially their speech volume, an AGC may be used to obtain an invariant audio impression for rear passengers irrespective of the actual speaker. Feedback suppression is generally needed to ensure stability of the closed-loop comprising loudspeaker, vehicle interior and microphone. The NDGC is used to optimize the sound quality for the listener, especially the volume of the playback signal. Additionally, the playback volume may be controlled by a limiter. Equalizing is required to adapt the system to a specific vehicle and to optimize the speech quality for the rear passengers.
These standard approaches are generally sufficient for unidirectional and some bidirectional systems. In state-of-the-art systems, typically only one noise-dependent module (NDGC) is used in each ICC instance to adapt the system to different acoustic scenarios. However, optimal performance of such a system is often not obtained when the number of acoustic zones/scenarios associated with the ICC instance is increased. Furthermore, particularly challenging is obtaining a consistent audio impression for each listener irrespective of the driving situation. Depending on the acoustic environment several psychoacoustic effects occur. Due to the Lombard effect, the speaker will change his voice characteristics to remain intelligible for the listener. On the other hand the speech signal played back from the loudspeaker will be masked by background noise at the listener's location. When speaker and listener are located in two different acoustic zones, the background noise may differ significantly so that these two effects may diverge. For example, the driver may increase the level of a fan in front of him, while a listener's fan remains switched off. A similar situation is given when the driver opens his window. In both cases the driver might speak louder than necessary so that the combination of direct sound and loudspeaker is inconvenient for the listener.