Speech control circuit

A speech control circuit for a telephone set having at least a microphone (M), a transmission switching circuit (SC.sub.T) coupled with the output of the microphone (M), a speaker (SP), a reception switching circuit (SC.sub.R), the output of which is coupled with said speaker (SP), a hybrid circuit (HYB) coupled with the output of said transmission switching circuit (SC.sub.T), said reception switching circuit (SC.sub.R) and a line, and a speech control circuit for controlling switching of said transmission switching circuit (SC.sub.T) and said reception switching circuit (SC.sub.R), characterized in that

• said speech control circuit comprises
• a transmission variable gain amplifier (VGA.sub.T) coupled with the output of said microphone (M),
• a transmission signal detecting circuit (SDC.sub.T) coupled with the output of said transmission variable gain amplifier (VGA.sub.T) for detecting a signal from said microphone (M) when the level of an amplified signal therefrom via said transmission variable gain amplifier exceeds a threshold level which is switchable between two levels,
• a first transmission time constant circuit (FTC.sub.T) for prolonging the rear edge of an output signal of said transmission signal detecting circuit (SDC.sub.T) by a first transmission time constant t.sub.t1,
• a second transmission time constant circuit (STC.sub.T) for prolonging the rear edge of the output signal of said transmission signal detecting circuit (SDC.sub.T) by a second transmission time constant t.sub.t2,
• a reception variable gain amplifier (VGA.sub.R) coupled with the output of said hybrid circuit (HYB) associated with the reception channel,
• a reception signal detecting circuit (SDC.sub.R) coupled with the output of said reception variable gain amplifier (VGA.sub.R) for detecting a signal from said hybrid circuit (HYB) when the level of an amplifier signal therefrom via said reception variable gain amplifier exceeds a threshold level which is switchable between two levels,
• a first reception time constant circuit (FTC.sub.R) for prolonging the rear edge of an output signal of said reception signal detecting circuit (SDC.sub.R) by a first reception time constant t.sub.r1, and
• a second reception time constant circuit (STC.sub.R) for prolonging the rear edge of the output signal of said reception signal detecting circuit (SDC.sub.R) by a second reception time constant t.sub.r2,
• said transmission switching circuit (SC.sub.T) being switched by the output signal of said first transmission time constant circuit (FTC.sub.T),
• said reception switching circuit (SC.sub.R) being switched by the output signal of said first reception time constant circuit (FTC.sub.R),
• the gain of said transmission variable gain amplifier (VGA.sub.T) being adjusted by the output signal of said first reception time constant circuit (FTC.sub.R),
• the gain of said reception variable gain amplifier (VGA.sub.R) being adjusted by the output signal of said first transmission time constant circuit (FTC.sub.T),
• said threshold level of said transmission signal detecting circuit (SDC.sub.T) being controlled by the output signal of said second reception time constant circuit (STC.sub.R),
• said threshold level of said reception signal detecting circuit (SDC.sub.R) being controlled by the output signal of said second transmission time constant circuit (STC.sub.T),
• said first transmission and reception time constants t.sub.t1 and t.sub.r1, being respectively longer than said second transmission and reception time constants t.sub.t2 and t.sub.r2.