Abstract: A voice-switched telephone circuit with center idle state improves bi-dirional gain control in loudspeaking telephones and thereby enhances the natural quality of interactive conversation. The voice-switched telephone includes: a transmit channel having a microphone and a three-state amplifier; a receiver channel having a three-state amplifier and a loudspeaker; and a hybrid circuit interconnecting the transmit and receive channels to a two-wire telephone line. The present invention is characterized by independent signal level detecting circuits for the transmit and receive channels. When the magnitude of the signal energy in the transmit channel exceeds a predetermined threshold, the transmit channel amplification is increased and the receive channel amplification is decreased by fixed equal amounts.

Abstract: A high voltage breakdown test circuit for testing of insulation resistance within an electrical device is disclosed. Operator safety is attained by allowing a fixed amount of energy considered nonlethal and advantageously imperceptible into the test circuit and by the internal system design. The system elements are connected in series and must all function properly in order to deliver the output signal to the device under test. Each system element obtains its power from the fixed energy input and is thereby activated only during an actual test. Indicator means of the pass/fail type provide an operator with a quick visual indication of the results of an insulation resistance test.