UBC Theses and Dissertations
A secure radio-frequency assistive listening device for hard of hearing people Chan, Paul S. K.
Assistive listening devices allow hearing impaired listeners to achieve higher comprehension than with hearing aids alone in difficult listening conditions which occurs during business meetings, lectures, and presentations. These devices minimize the acoustic pathway between the speaker and listener by using some form of electromagnetic transmission. A major problem with existing assistive listening devices is that they provide little security for the users, as the signal may be picked up by unauthorized listeners outside the confines of the meeting room or hall. The design, implementation, and testing of a secure radio-frequency assistive listening device are described in this thesis. The device combines digital voice encoding technology with the direct sequence spread spectrum technique to provide a secure digital voice communication channel for hearing impaired people. It is shown that the CCITT 0.722 and the digital voice encoding algorithms are acceptable for hearing impaired listeners. A transmitter and receiver based on the Arlan 650Tm wireless network card is implemented. Digitized speech samples are grouped into packets and transmitted using the TCP/IP protocol. The packet error rates of the device measured under some typical environment are presented. The degradation of intelligibility under packet loss conditions is also presented. It is shown that speech intelligibility decreases as the number of lost speech segments increases and the lost speech segments are replaced by two simple packet loss replacement methods.
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