Application of Frequency Shifting in In-Car Communication Systems

A known approach for stabilizing acoustic feedback loops is to include time-varying processing in the signal path. Therefore, we present a subband-based implementation of a phase modulation filter that allows arbitrary frequency shifts, comment on some limitations of the method, and discuss practical implementation aspects. Listening tests are conducted in order to derive the parameter range in which the introduced frequency shift is not audible to listeners. Special test scenarios are set up to cover the intended use of the algorithm in an in-car communication (ICC) system and confirm that background noise and the direct speech components are masking the frequency shift to some extent. Experiments with an ICC system in a test car show that persistent howling occurs only after considerable increase of the system gain, but also strong artifacts are introduced. This leads to the conclusion that in the ICC context, the proposed method is particularly suited for signal decorrelation which is an important part of techniques for feedback prevention.