Transcrypt International, Inc.

David L Dilley

James R Holthaus

An apparatus and method for providing proper bias current and load resistance for a telephone microphone regardless of the type of microphone used. Microphone bias voltage, if any exists at the microphone, is compared to a reference voltage selected to define a distinction between various types of microphones. If the bias voltage exceeds the reference voltage, an impedance matching the correlated type of microphone is interdicted on both sides of the microphone. If reference voltage exceeds bias voltage, a different impedance path is presented to match it with the type of correlated microphone. Therefore, the invention automatically provides proper bias current and load resistance for the microphone.

Apparatus and method for providing proper microphone DC bias current and

An apparatus and method for determining the presence and polarity of DC bias voltage at a telephone microphone. A circuit can be added in parallel between any hand or headset and a telephone base unit. The circuit converts the DC bias voltage, if any, across the microphone into a pulsed signal. Each pulse charges a capacitor. The output of the capacitor is passed through a resistor transformer network, and sampled. In between each pulse, the capacitor is discharged and the resulting signal is passed through the transistor/resistor network and sampled. By determining the averages of the charging and discharging signals, and sampling them to convert it into digital information, a digital signal processor can derive whether any bias voltage is present across the microphone, and, if so, its polarity. The circuit can be electrically isolated from the microphone and telephone.

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Ft. Collins, CO, United States of America

David L Dilley