Abstract: Device for performing the "hands-free" function in a telephone set associating the gain switching and echo cancellation functions. This device is characterized in that it comprises a gain switch and an echo canceller, said switch having in the emission and reception channels a circuit for measuring the amplitude of the signals present on these channels, a checking circuit for determining the state of the telephone set (emission, reception, rest,) a calculating member controlled by the checking circuit, variable attenuators controlled by the calculating member, said calculating member evaluating, as a function of the relative amplitudes of the signals in emission and in reception, the echo cancellation performances obtained by the device and adjusting as a function thereof the variable attenuators in each channel, so that the overall attenuation in the loop always assumes the minimum value ensuring the stability of the system.

Abstract: The protective device affords electrical insulation at all times between a terminal and a telephone line, without any modification to the terminal, and protects a subscriber against any voltage surge (resulting from lightning, for example), and transmitted in the line. The device includes optical guides bidirectionally transmitting in particular voice, multifrequency and data signals and providing insulation between two interfaces respectively connected to the terminal and the line. One interface is fed with a remote supply current and the other is fed from the main power supply. The protective device can further include moving insulating rods for driving switching devices for the terminal to seize the line and for reproducing an interrupted ringing signal transmitted from the line to the terminal.