Phlipp Bulling: Rückkopplungsunterdrückung für Innenraumkommunikationssysteme
- Prof. Dr.-Ing. Gerhard Schmidt
- Prof. Dr.-Ing. Jürgen Freudenberger
- Prof. Dr.-Ing. habil. Thomas Meurer
- Prof. Dr.-Ing. Michael Höft
- Prof. Dr.-Ing. Jeffrey McCord
(head of the examination board)
The communication between the passengers inside a car can be difficult due to large background noise levels. It can be improved with so-called in-car communication systems. These systems capture the voice of talkers by means of microphones and play it back via loudspeakers close to the listeners. However, the challenge is the electro-acoustic feedback, which occurs when the microphone not only captures the local speech but also the loudspeaker signal. Without countermeasures, this feedback results in annoying howling sounds.
The problem of the electro-acoustic feedback has not yet been solved for in-car communication systems. Therefore, in this work techniques to suppress the feedback by means of digital signal processing are presented. The main part of this work focuses on adaptive feedback cancellation. Here, the impulse response between loudspeaker and microphone is estimated with an adaptive filter. The difficulty is a strong correlation between loudspeaker and local speech that prevents the adaptive filter from converging towards the desired solution. In order to improve convergence, a novel stepsize control is presented. As signals are not correlated during reverberation, the stepsize control exploits reverberant signal periods to update the filter coefficients. In addition to the adaptive feedback canceler, a postfilter is presented. The task of the postfilter is to suppress the residual feedback that remains after the feedback cancellation, by means of a Wiener-filter. Therefore, the postfilter is controlled depending on the adaptive filter's state of convergence. Finally, two techniques to improve the speech quality are presented. Firstly, an automatic equalizer is described that improves the sound quality. Secondly, it is shown that speech intelligibility can be improved by adding harmonics to a speech signal.
Besides the theoretical investigations, in this work also the practical realization of the algorithms is regarded. Therefore, the algorithms are integrated into a specially developed real-time framework and tested in demonstration cars under realistic conditions during numerous test drives. These test drives show a significant increase of both stability and speech quality compared to existing approaches.