For a voice-controlled telecommunication terminal designed at the Forschungs- und Technologiezentrum of Telekom, a comfortable hands-free unit has been developed and implemented on two DSP-boards. Because of the well-known disadvantages of conventional speakerphone circuits this hands-free unit contains, as a central element, a canceller for acoustic echoes. This canceller is implemented as an adaptive FIR filter with approximately 1000 coefficients. A sampling rate of 8 kHz assumed, this allows more than 120 ms of an impulse response of a room to be cancelled. The coefficients of the canceller are adjusted by an NLMS algorithm supported by adaptive prewhitening filters. A special stepsize control renders the canceller robust against disturbing signals. As part of the hands-free unit, moreover, a canceller with 50 coefficients for the echoes of the hybrid and a weighting balance have been implemented. The balance is only intended to add the necessary attenuations which are not provided by the cancellers in order to ensure a stable and comfortable hands-free operation at any time.

Keywords: Voice Controlled Telecommunication Terminal, Hands-free Telephones, Echo Cancellation, Speech Recognition.