Dan Greenwood

Publication Date


Document Type


First Advisor

Kuo, Sen M. (Sen-Maw)

Degree Name

M.S. (Master of Science)

Legacy Department

Department of Electrical Engineering


Telephone--Equipment and supplies; People with disabilities--Means of communication


Digital signal processing techniques can be used to implement a hands-free telephone for persons with disabilities which allows the user to initiate or answer telephone calls without manual dialing. The functions of this system include speech recognition, speech synthesis, and the operations required for an actual hands-free telephone for use in either emergency and non-emergency situations. In this thesis, the design and implementation of the system hardware and communication software are presented. An analysis of the closed loop system which is created in a hands-free telephone system is presented . Particular interest is given to the two echo paths which exist in this type of system. A closed loop gain analysis is performed to provide insight into the instability inherent in this type of telephone. Once the system is understood, a solution is proposed for the telephone system for persons with disabilities. Included in the solution are two echo cancellers, voiced switched attenuators, speech detectors, and a closed loop controller algorithm. Simulation results for the two echo cancellers are presented and explained. Once a system level solution has been developed, the system hardware is presented and discussed. The actual performance of the system is shown with particular interest paid to the echo cancellers. Finally, a proposal for areas of continuing research for this telephone system is presented. The hardware implemented in this thesis has the capability of being easily expanded to provide several innovative features to the system. The goal of this thesis is present a solution for the hands-free telephone communication problem as well as a hardware platform for future enhancements. The complete system provides features which will allow disabled persons to easily interface with others using the existing telephone network while also having the capability of being expanded to provide such features as environmental control and home security.


Includes bibliographical references (leaf 78)


xi, 89 pages




Northern Illinois University

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