Patent GB-567694-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04M9/00
Filed: 1943-08-19
Abstract: 567,694. Intercommunication systems. DICTOGRAPH TELEPHONES, Ltd., and WEBB, S. Aug. 19, 1943, No. 13507. [Class 40 (iv)] In an intercommunication system in which the talking circuit is made over the home lines of the calling and called stations, the actuation of a selecting key at the calling station causes the identity of that station to be indicated visually at the called station, and the appropriate selecting key at the called station must be actuated in order to complete the talking circuit. As shown, the lines comprise twisted pairs, and the receiver 11 of each station is connected across its home line, while the microphone transformer XFR may be connected to any other line by the operation of relays KRa, KRc ... controlled by the keys PKa, PKc ... The battery feed for the microphone 14 is dependent on relay HLR which is controlled over the home line by the keys at other stations. The main battery which supplies the microphone current over -, T+ also provides a higher potential over R+ for lighting lamps 13 and for operating relay LF, while a second battery supplies ringing current over Z+, Z-. To make a call, the appropriate key PK is fully depressed to close all its contacts and then released, whereupon it assumes an intermediate position in which only contacts 1, 2 are closed until the hand-set is replaced. Relay KR energizes in series with relay SHR, which disconnects the buzzer 10 and bridges the receiver 11 across the Z battery to pick up ringing tone, and connects T+ to the called line to operate relay HLR at the called station. Ringing relay RKR energizes momentarily over contacts 3, 4 and connects R+ to the called line to operate relay LF and Z+ to the home line of the calling station to energize the appropriate relay LR at the called station, and this relay locks in series with lamp 13 and cut-off relay COR which disconnects LF and, unless the station is busy, closes the buzzer circuit. To reply, the called party actuates the key corresponding to the lighted lamp. Relay KR restores the calling circuits and brings about the operation of relay HLR at the calling station, but relay RKR is inoperative. The system may include a master station, Fig. 2 (not shown), which is provided with a loudspeaker and amplifier. To enable the master station to call an ordinary station when its cut-off relay COR is up, the ringing relay momentarily connects Z- to the negative of the main battery in order to operate relay LF, which in this condition is connected to Z -. For conference facilities, an additional line may be provided to which all the stations may become connected. The Provisional Specification describes a generally similar system in which the lines are single wires, and in the talking condition the instruments at the calling and called stations are transformer-coupled to a circuit over both home lines. The microphone of the master station, however, is connected over a separate line to the receiver at an ordinary station in order to avoid interaction. The duration of the ringing operation may be limited by the charging of a condenser.