Speech intercommunication system

An intercommunication system comprising

• a transmit channel and a receive channel, said transmit channel including a microphone operative in the transmit mode and a first variolosser circuit, and said receive channel including a loudspeaker operative in the receive mode, a manually gain-adjustable circuit, a second variolosser circuit and an automatic gain control amplifier, said first and second variolosser circuits each providing a variable amount of loss and permitting the high frequency region of the voice frequency band to pass therethrough including a mode wherein the amount of loss is relatively large,
• a hybrid transformer connecting said transmit and receive channels,
• first means for deriving a firt control signal from said transmit channel,
• second means including a switch for selectively deriving a second control signal from said receive channel, said second control signal being derived in the transmit mode from a point in said system adjacent said hybrid transformer and in the receive mode from a point in said system adjacent said loudspeaker,
• subtraction means for subtracting said first control signal from said second control signal,
• first and second comparators having first and second different threshold values coupled to the output of said subtraction means, said first threshold value corresponding to the second control signal derived adjacent said loudspeaker and said second threshold value corresponding to the second control signal derived adjacent said hybrid transformer, and
• a flip-flop having first and second inputs coupled to the outputs of said first and second comparators, respectively, and first and second outputs coupled to said first and second variolosser respectively, said flip-flop selecting the variable amount of loss in said first and second variolosser in accordance with the outputs of said comparators.