Speech-amplifier telephone station

Claim 1

A speech-amplifier telephone station comprising a first attenuator inserted in a transmission channel for connecting a microphone to a circuit which provides a coupling with a telephone line, a second attenuator inserted in a reception channel for connecting said coupling circuit to a loudspeaker, means for controlling said attenuators as a function of the relative levels of the signals collected respectively upstream of said attenuators in said transmission and reception channels, said levels being calculated respectively by integrators, and means for introducing a difference between the levels of signals collected respectively from said channels once said attenuator associated with one of the channels has been put in an attenuation state and said attenuator associated with the other channel has been put in a non-attenuation state, each attenuator aforesaid being constituted by a variable impedance which is mounted in shunt on the corresponding channel and is capable of assuming two values, namely a very low value and a very high value, said means for introducing a difference between the levels of the signals collected respectively from said channels consisting of a resistor bridge formed by means of two fixed resistors inserted in each channel aforesaid upstream of said attenuator which is associated with said channel, and by means of said variable impedance which constitutes said attenuator, the common node of said two fixed resistors being intended to constitute the point of collection of the signal from each of said channels, each attenuator aforesaid being constituted by two diodes mounted in the same direction and having a common electrode connected to said channel which is associated with said attenuator, the other electrode of one of said diodes being put at a given potential whilst the other electrode of said diode is connected to one of the terminals of a current generator which is capable of generating a current having a first or a second value according to the control applied thereto by said control means, said diodes being such as to have a very low resistance or a very high resistance according to the current which flows through said diodes at said first or said second value, the other terminal of said current generator being put at a given potential, one of said two given potentials being intended to constitute a reference potential.