A frequency multiplexed system for coupling a plurality of individual telephone sets to at least one subscriber telephone line. The system includes a line adapter unit for controlling use of subscriber lines and for assigning the use of frequency bands to the individual telephone sets. The system includes a two-conductor interconnection cable for coupling to each of the telephone sets and to the line adapter unit. The system has a power-down mode in which the loss of external power allows each of the individual telephone sets to use the subscriber line. The line adapter unit and each of the individual telephone sets employ transconductance amplifiers so that modulated current signals are produced on the interconnection cable, thus allowing a plurality of voice signals to be summed together by voltage addition for conferencing between the telephone sets and the subscriber lines. One embodiment of the invention includes the production of a pilot reference signal on the interconnection cable by the adapter unit so that synchronous modulation and demodulation by each of the telephone sets is made possible.

Frequency multiplexed telephone system

Within a key telephone system, there is provided a circuit for completing a direct signalling path from a first station to another station in the system. By depressing a button at the first station designated with identifying indicia of the other station, a direct speaking connection is made from the first station to the other station. The first station may be an attendant's position with direct key appearance for each station to which the attendant is to have direct access. In a preferred form, the called station has a loudspeaker associated with the station instrument. In this instance, a direct call results in a signalling tone directed to the loudspeaker, following which the attendant can converse with the speaker-equipped station. With an attendant's position, depression of a direct station key automatically places a line to which the attendant had been connected on hold.

Direct station selection circuit

A telephone intercommunication system where a plurality of stations have access to a plurality of lines. Each station has a pushbutton type keyset which is connectible to integrated circuits associated with each line and are utilized and generate signals for controlling central office switching equipment.

Telephone intercommunication system incorporating improved dialing arrangement

A private telephone intercommunication system including facilities for connecting the system to a telephone central office. Separate communication channels and supervisory means are provided for external and internal calls.

Telephone intercommunication systems

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Telephone intercommunication systems