Receive Side Processing for Automotive Hands-Free Systems

In the sending path of automotive hands-free systems several subunits - such as acoustic echo cancellation (AEC) and noise reduction (NR) - improve the quality of the outgoing signal. These units are usually realized in the frequency or subband domain in order to reduce the computational complexity. In the receiving path, however, only a few signal processing stages - such as bandwidth extension (BWE) [1] or gain adjustment - are realized in recent systems [2, 3]. These units are implemented in most cases in the time domain, since two analysis-synthesis schemes (one in the sending and one in the receiving path) would introduce more delay than allowed by ITU- or VDA-recommendations [4]. According to the best knowledge of the authors linking of conventional processing schemes in the sending path (AEC and NR) with those of the receiving path has not yet been addressed in research on hands-free systems. For the car environment some amplifier manufacturers perform a volume control in dependence of the driving speed of the car. Some have even the possibility of placing a microphone in the cabin for measuring the noise level within the car [2, 5]. But this does not apply to hands-free telephony. The estimated power spectral density (PSD) of the background noise (already estimated within the NR unit) can be used to adjust the BWE unit. Since in high noise conditions, artifacts introduced by a BWE scheme are less audible a stronger extension can be used compared to stand-still operation. Taking also the estimated echo spectrum into account (beside the noise PSD) an estimate for the SNR within the car cabin can be obtained. Using this estimate one could perform an automatic gain control of the receive signal for retaining a particular SNR within the car while the noise or the speaking level of the remote partner is changing. This can also be done in a frequency specific manner, resulting in a frequency selective adaptive equalization. No further microphone has to be placed in- the cabin and the volume can be controlled independent of the amplifier using the resources (AEC, NR) already available.