Abstract: To allow a universal replacement telephone handset, an adaptive amplifier circuit uses DC power available from a host telephone set base to discriminate between low resistance and high resistance base power supplies. It emulates either a carbon microphone or an electret microphone, whichever is appropriate for the set base. The circuit couples a microphone (10) and an output terminal (2a) through a series connection of resistor (R1), input and output leads of an amplifier (12), and a sense circuit (22). The sense circuit controls two current sources (14),(16) in response to the shunt voltage and series current available from the telephone base. Current source (14) connects the amplifier output to an input terminal (1a) and acts either as a bias current source, or as an electronic switch that provides a short circuit from the microphone to the the amplifier output.

Abstract: A voice-band receive amplifier for telephone handsets that mitigates loud sidetone and acoustic sing. A receive channel (30) with amplification and voice-switched attenuation is disposed between a receive port of a telephone network (80) at terminals (1a), (1b), and an electro-acoustic receiver (20). A transmit channel (70) having voice-switched attenuation that varies inversely with the receive channel attenuation is disposed between a transmit port of the telephone network at terminals (2a), (2b), and an acousto-electrical microphone (10). A controller (60) with an output lead connecting to the transmit and receive channels has an input lead connecting to the microphone, and an input lead connecting to the receiver. The controller provides a varying DC control signal at its output lead in response to varying levels of transmit and receive signals appearing at the input of the controller. Receive signals are attenuated when transmit levels are at or above a predetermined threshold.

Abstract: An off-hook/on-hook status check circuit is adapted to be connected in series with either the tip (2) or the ring lead (3) of a telephone subscriber loop (1) at a location remote or apart from the telephone station set (13). The circuit comprises an opto-emitter subcircuit (7) in series with a capacitor (4). These are connected in parallel with a pulsing contact (K.sub.1) which alternately permits the charging of the capacitor by the telephone central office voltage or its discharge through the opto-emitter subcircuit. As the capacitor may only be charged when the telephone station set is off-hook drawing loop current, the opto-emitter subcircuit provides a predetermined output when the capacitor discharges, identifying the off-hook condition of the telephone station set. No output from the opto-emitter subcircuit during the capacitor discharge interval identifies the on-hook condition of the set.

Abstract: In order to prevent excessive voltages originating in customer owned terminal equipment from creating a hazardous condition in an associated telephone line, a protective circuit is connected between the equipment and the line. This circuit responds to an unbalanced current flow by disconnecting the equipment from the line and providing a local path to ground for the current. The circuit remains activated as long as the unbalanced current flow continues and reconnects the equipment to the line only when the current flow ceases.