TELEPHONE SUBSCRIBER INSTRUMENT

摘要

Preamplifiers and amplifiers are used for transmit and receive channels respectively and power supplies are provided for ancillary circuit. The outputs of the preamplifiers and the inputs for the amplifiers are coupled to external terminals so that the channels may be coupled directly or via an ancillary circuit for hands free operation. The preamplifiers gains are such that the signals on the channels may be compared. This provides selection of one or other channel by the ancillary circuit. - An ancillary chip has a tone generator coupled to a loudspeaker amplifier. The output of the receiver channel amplifier is selectively coupled to the loudspeaker amplifier. A second ancillary chip has two attenuation channels for the received and transmitted signals and a control for the attenuation loss in each channel. The attenuation channels are coupled between the primary chips amplifiers and preamplifiers to provide a variable attenuation path. The control prevents acoustic coupling between the receiver and transmitter channels.