A prototype, air conduction, screening audiometer has been developed utilising a 16-bit fixed point digital signal processor. Circuit functions such as sine wave generation, envelope shaping and attenuation are performed in the digital domain. The analog signal, generated by a sixteen- bit digital to analog converter, is passed through a filter and amplifier stage before being connected to the right or left earphone via electronic relays. The instrument is capable of operating in three basic modes. The manual mode allows test tone parameters to be controlled by simple push buttons with an LED display indicating the instrument settings. The automatic mode carries out a Hughson-Westlake type of test under the control of the patient with the test results available as a tabulated printout. The third mode allows remote control of the instrument via an RS232 interface. No manual setting up of circuitry is required and calibration is enabled by a single switch. Calibration offsets and various programmable parameters of the audiometer are stored in a non-volatile memory. Comprehensive measurements were carried out on the prototype audiometer by a specialist audiometer calibration laboratory.
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