Voice switched speakerphone with filters having exponential transfer function in the monitoring path

A handsfree telephone circuit with separate transmit and receive audio paths for connection to a microphone and speaker respectively, comprising a controllable attenuator in each of said transmit and receive paths;

• attenuator control means responsive to level control signals for incrementally setting the attenuation level of said attenuators, said control means responsive to a directional control signal for determining which audio path at any moment is a controlling path having control of said attenuators and which audio path is a non-controlling path;
• a pair of monitoring circuits for generating said level control signals, said monitoring circuits defining respective monitoring paths connected to each of said transmit and receive paths for sensing audio signals therein, each said monitoring circuit comprising a high pass filter for attenuating background noise, a rectifier, a peak level detector, and a speech detector;
• a pair of low pass filters having a fast attack and slow decay with a transfer function of approximately inverted exponential form connected to the outputs of the respective peak level detectors in the two monitoring circuits;
• and a switching comparator for comparing the outputs of said low pass filters to produce said directional control signal for said attenuator control means;
• and means for disabling the low pass filter receiving the output of the peak detector in the non-controlling audio path, the output of said low pass filter in the controlling audio path preventing the switching comparator from outputting the directional control signal upon echo signals appearing in the non-controlling path.