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.