Control device for a hands-free telephone set

Abstract

A control device for a hands-free telephone set automatically controls microphone and amplifier gains so that a feedback loop has less than unity gain to avoid circuit instability and resultant self-oscillation. An emission channel includes a microphone, a signal compressor and a controllable attenuator. A reception channel includes a signal compressor, an adjustable attenuator and a loudspeaker. The combination of the emission and reception channels form an amplification loop whereby the output of the reception channel is acoustically coupled to the input of the emission channel while the output of the emission channel is coupled to the input of the reception channel through a common telephone line. To avoid circuit oscillation, a circuit initially sets the gain of the loop to a predetermined value slightly less than unity (0 db) and subsequently maintains the loop gain constant by maintaining the sum of the compressor and attenuator gains at a fixed value.

Description

BACKGROUND OF THE INVENTIONThe present invention generally relates to a control device for a hand-free telephone set. This device comprises an emission channel connected at one extremity to a microphone and at the other extremity to a telephone line interface and a reception channel connected at one extremity to the line interface and at the other extremity to a loudspeaker.Such an apparatus comprises an amplifying loop constituted, on the one hand, by the two emission and reception channels and, on the other hand, by the loudspeaker / microphone acoustic coupling and by the electric coupling generated by the line interface which connects the emission and reception channels of each apparatus to the telephone line. If the gain of this amplifying loop is higher than one, an instability or Larsen effect occurs, which produces an unpleasant hiss.In order to avoid this parasitic effect, an attenuator is generally disposed in each of the emission and reception channels.An exemplary system, ...

AI Technical Summary

Benefits of technology

is to palliate this drawback and reduce this adverse effect during sw

Problems solved by technology

If the gain of this amplifying loop is higher than one, an instability or Larsen effect occurs, which produces an unpleasant hiss.

In other words, it is not possible for those two parties to speak simultaneously since only one of the two channels of each telephone set is active, the emission channel of the talker and the reception channel of the listener.

In such a case, there will always appear a maximum attenuation on the non-noisy channel and the other party could never be able to speak.

This prior art device supplies satisfactory results as regards selection of the active channel and elimination of noise signals.Iadd., .Iaddend.but still presents some drawbacks during switching from one channel to another.

As a result, during switching of the emission and reception channels, the transition is long if the switching time constants are chosen too high or causes an unpleasant parasitic effect for the listener.

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