Eberhard Hänsler

The hands-free telephone problem: an annotated bibliography

Abstract Providing means for a ‘comfortable’ hands-free telephone conversation is a long standing problem. Comfortable in this context means full duplex conversation between speakers moving freely in an ordinary office or living room. Currently available solutions to the ‘hands-free’ problem either rely on a studio environment or employ voice-controlled switches. In the first case, decoupling of loudspeaker(s) and microphone(s) is achieved by using highly directional devices; reflections are prevented by sound absorbing enclosures. In the second case, speech controlled loss devices are inserted into the communication loop in order to attenuate the signal in the direction showing least activity. Thus, only half-duplex communication is provided. The concept of a comfortable solution is extremely simple; if an exact (electronic) replica, parallel to the loudspeaker-enclosure-microphone system (LEMS) is provided, the decoupling of loudspeaker and microphone can be achieved without any restrictions to the electro acoustic system and - more importantly - without any loss of convenience to the user. The implementation of such a system is extremely complicated, however. The difficulties are due to three factors; the impulse response of a LEMS may have a duration of several hundred milliseconds, the system has to be adaptive, and the adaptation has to be performed with a speech input. This annotated bibliography summarizes the attempts to solve the ‘hands-free’ telephone problem.

Hands-free telephones: joint control of echo cancellation and postfiltering

Abstract This paper deals with an efficient and robust joint control of the step sizes of subband adaptive echo compensation filters and the frequency response of the echo suppression filter of a hands-free telephone system. It is shown that both control methods are described by the same quantity: the ratio of the short-term estimates of the power of the error to the “undisturbed” error signal. Since the latter is not directly measurable, a three-step procedure for its estimation is given. It uses speech activity detection, a “shadow” filter, and a correlation analysis. The procedure described has been implemented. It produced a considerable reduction in the amount of necessary signal processing. Furthermore, it was possible to provide natural communication with hands-free telephone systems.

The hands-free telephone problem: an annotated bibliography update

Providing means for a full-duplex hands-free telephone conversation is still a research topic in many communication and signal processing laboratories around the world. More than ninety publications mainly from the last two and a half years support this statement. The reason for these activities lies in the complexity of the problem: full-duplex telephone communication without hand-sets requires electronic replicas of the loudspeaker-enclosure-microphone systems used by the communicating parties. The impulse response of such a system is typically several 100 ms long and time varying. Therefore, the replica has to be adaptive while the adaptation is controlled by speech signals. Regarding these requirements the hands-free telephone problem may currently be considered as one of the most challenging signal processing problems. This bibliography supplements a bibliography composed approximately two and a half years ago [29]. Brief summaries are given on papers published since then.RésuméL’étude des techniques permettant la communication mains-libres en duplex est toujours un sujet de recherche dans de nombreux laboratoires travaillant dans les télécommunications et le traitement du signal de par le monde. Cette observation est étayée par l’existence de plus de quatre-vingt dix publications parues principalement depuis deux ans et demi. La communication téléphonique duplex sans combiné nécessite la modélisation du système constitué par le haut-parleur, la salle et le microphone à chaque extrémité. La réponse impulsionnelle de tels systèmes a une longueur typique de plusieurs centaines de millisecondes et varie au cours du temps. Par conséquent, le modèle doit être adaptatif, l’adaptation étant contrôlée par le signal de parole. Compte tenu de ces diverses contraintes, la téléphonie mains-libres peut être considérée actuellement comme l’un des problèmes les plus difficiles en traitement du signal. La présente bibliographie complète une bibliographie établie il y a environ deux ans et demi [29] ; elle donne un bref résumé des articles publiés depuis lors.

Adaptive echo compensation applied to the hands-free telephone problem

The requirements of a loudspeaker-enclosure-microphone system model are reviewed. It is shown that decoupling of the loudspeaker and the microphone requires an adaptive system with an impulse response of several-hundred-microsecond duration. Means to solve the adaptation problem are described.<<ETX>>The requirements of a loudspeaker-enclosure-microphone system model are reviewed. It is shown that decoupling of the loudspeaker and the microphone requires an adaptive system with an impulse response of several-hundred-microsecond duration. Means to solve the adaptation problem are described. >

The hands-free telephone problem

The author describes the problem of providing the means for a comfortable hand-free telephone conversation. After a report on conventional schemes the properties of a loudspeaker-enclosure-microphone system are discussed. It is shown that for a truly two-way conversation the decoupling of the loudspeaker and the microphone requires an adaptive filter with an impulse response of several hundreds of milliseconds in duration. The attempts to solve the adaptation problem are outlined. >The author describes the problem of providing the means for a comfortable hand-free telephone conversation. After a report on conventional schemes the properties of a loudspeaker-enclosure-microphone system are discussed. It is shown that for a truly two-way conversation the decoupling of the loudspeaker and the microphone requires an adaptive filter with an impulse response of several hundreds of milliseconds in duration. The attempts to solve the adaptation problem are outlined.<<ETX>>

Single-Channel Acoustic Echo Cancellation

This chapter explains the problem of acoustical echoes and their cancellation. It focuses on the hands-free telephone as one of the major applications for echo cancelling devices. Beginning with a brief discussion of a system formed by a loudspeaker and a microphone located within the same enclosure, the properties of speech signals and noise are described. The major part of the echo can be cancelled by an adaptive filter connected in parallel to the loudspeaker and the microphone. Residual echoes may be suppressed by an additional filter within the outgoing signal path. We will address only single-channel solutions, which means that we will have a monophonic loudspeaker signal.

Acoustic Array Processing for Speech Enhancement

This chapter contains sections titled: Introduction Signal Processing in Subband Domain Multichannel Echo Cancellation Speaker Localization Beamforming Sensor Calibration Postprocessing Conclusions References

Chapter 8. Acoustic Array Processing for Speech Enhancement

This chapter contains sections titled: Introduction Signal Processing in Subband Domain Multichannel Echo Cancellation Speaker Localization Beamforming Sensor Calibration Postprocessing Conclusions References

Algorithms for Adaptive Filters

This chapter includes the following topics: The Normalized Least Mean Square Algorithm The Affine Projection Algorithm The Recursive Least Squares Algorithm The Kalman Algorithm ]]>

Control of Noise and Echo Suppression Systems

This chapter includes the following topics: Estimation of Spectral Power Density of Background Noise Musical Noise Control of Filter Characteristics