Phone with adjustable sidetone

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 switch 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;
• and wherein said switch is a transistor having an on state wherein a low resistance path is provided through said transistor from said output of said transmit amplifier to said resistor and an off state wherein said path from said output of said transmit amplifier to said resistor has high resistance,
• and further comprising a microcontroller programmed to control said transistor to be in either said on state or said off state depending the state of configuration data.

A process for controlling sidetone levels in a telephone, comprising the steps of:

• converting sounds into audio signals and coupling them through a transmit amplifier of a hybrid circuit to tip and ring lines of a telephone line for transmission to another conversant in a telephone conversation, wherein said hybrid has a close impedance match with the characteristic impedance of said tip and ring lines so as to generate little or no sidetone;
• receiving audio signals from said tip and ring lines and coupling them through said hybrid circuit to a summing node virtual ground at the input of a differential amplifier having a low pass filter feedback path, which in conjunction with resistors coupled to said summing node, set the gain of said differential amplifier, said differential amplifier driving an audio transducer for conversion of said audio signals to audible sound;
• and
• feeding back part of the audio signal at the output of said transmit amplifier to said summing node through a sidetone only feedback path comprised of a switch and resistor and closing said switch when a higher level of sidetone is desired and opening said switch to cut off said feedback path when a lower level of sidetone is desired.