Evaluation of Hands-free Terminals

The “hands-free problem” describes the high acoustical coupling between the hands-free loudspeaker and microphone and the resulting acoustical echo for the subscriber on the far end of this connection. It is basically caused by the high distance between the technical interface, i.e. hands-free loudspeaker and microphone, and the human interface, i.e. mouth and ear. The high playback volume – necessary to provide a sufficient playback level at the users ear – and the high sensitivity of the microphone – necessary to amplify the users voice from far distance – leads to a strong coupling – the acoustic echo. Echo cancellers instead of level switching devices are standard today. Moreover, noise reduction and other algorithms further improve the speech quality in noisy environment. On the other hand the use of mobile hands-free telephones in a wide and important application field, i.e. in vehicles, was further enforced by legislation. Hands-free telephones are standard in the automotive industry – at least in middle and upper class vehicles. As a consequence the test procedures and results described in this section mainly focus on the test of hands-free terminals installed in vehicles.