Patent ID: 6151391
Filing Date: 2000-11-21
Classification: H04M

Abstract:
A circuit with selectable sidetone levels in a telephone, comprising:tip and ring lines for receiving and transmitting audio signals, said tip and ring lines having a characteristic impedance;a three port hybrid circuit having a first port coupled to said tip and ring lines and having an audio input port for receiving audio signals to be transmitted, and having an audio output port, and including a transmit amplifier to amplify signals received at said audio input port and having a hybrid impedance coupled between said first port and the output of said transmit amplifier, and having a receive amplifier coupled to amplify a difference voltage generated from the difference between the voltage at said output of said transmit amplifier and the voltage at said said first port and present the amplified signal at said audio output port;an audio transducer such as a speaker for converting audio signals to audible sounds;a differential amplifier having inverting and noninverting inputs, one of said inputs coupled to a reference voltage and the other of said inputs coupled to said audio output port of said hybrid and acting as a summing node, and having an output coupled to drive said audio transducer;a low pass filter coupled to provide feedback of part of the ouput signal generated by said differential amplifier back to said summing node; and a programmable sidetone only feedback conductive path comprised of a manually operable switch accessible to the user of said telephone and a resistor, said switch having an input coupled to said output of said transmit amplifier and having an output coupled to one of two terminals of said resistor, the other terminal of said resistor coupled to said summing node of said differential amplifier driving said audio transducer.