Decentralized local telephone system

Claim 1

In an intercommunication system, the stations of which contain at least one electro-acoustic transducer, a duplex amplifier and connecting devices for the establishment and release of connections using a small number of common pairs of conductors comprising communication lines to which the stations are connected in parallel; the improvement comprising a common control circuit which contains a first pulse generator means which, at regular intervals, repetitively transmits along a pair of conductors common to all stations a pulse train consisting of a number of equal pulses and with a pause before the next pulse train, said pulse train being fed to pulse receiving means at each station operative to provide the pulses of the train sequentially to a number of selector keys in each station, this number of selector keys being equal to one less than the number of pulses in said pulse train, lined seizure circuit means in each station connected to receive and utilize the final pulse of each pulse train in the seizure by a station of a free communIcation line, line unit means for each communication line connected to a station and to said control circuit, said line unit means including second pulse generator means for providing during the pause between pulse trains a respective line pulse which is of a phase characteristic of a communication line, said line seizure circuit means operating when a line pulse is received at the same time as a calling pulse to connect a station to the communication line represented by this line pulse as a calling station, calling means in each station operative upon connection of said station to a communication line and upon activation of a first selector key to supply a first connecting pulse digit signal from said pulse train through said first selector key to said communication line and upon activation of a second selector key to provide a second digit pulse signal subsequent to said first digit pulse signal to said communication line, and each station connected to said communication line including identification circuit means connected to receive first and second identification pulses from two of said keys, and receiver control means connected to receive said first and second identification pulses and said first connecting pulse digit signal and second digit pulse signal from said communication line, said receiver control means operating when said first connecting pulse digit signal is received in coincidence with said first identification pulse to permit access, for a fixed period of time, to said second digit pulse signal and upon the coincidental reception of said second digit pulse signal and second identification pulse to establish communication over said communication line with said calling station.