Patent GB-677911-A (GB)
Title: Improvements in or relating to intercommunication telephone systems
Inventor: IVENS EDGAR
IPC: H04M9/00
Filed: 1949-09-16
Abstract: 677,911. Intercommunication systems; loudspeaker systems. IVENS, E., and MODERN TELEPHONES (GREAT BRITAIN), Ltd. Sept. 15, 1950 [Sept. 16, 1949], No. 23963/49. Class 40 (iv). In an intercommunication system comprising master stations and substations, the substations comprise a transducer S which may be used as a loud-speaker or a microphone, and communication is established through an amplifier A the effective direction of which is controlled by a relay AM in response to a key TK at a master station. The system, Fig. 4, comprises master stations M having a telephone hand-set H and master stations LM comprising in addition a loud-speaker LS and a microphone m for non-amplified inter-master communication. Hand-set master to hand-set master, say MI to MII.-Operator at MI depresses the second button of a push-button strip PB to energize by battery R a buzzer BZ at the station MII. The call button on release disconnects the lower ringing bar of the switch PB and is held mechanically in contact with the upper bar to provide the talking circuit over a condenser C at the calling station and the back contacts of relays PR at both stations. The antiside-tone bridge and feeding circuit is of the kind described in Specification 650,070, power being provided by a battery T. Replacement of the hand-set at the calling station finally releases the calling button. Hand-set master (M1) to substation (S1).- The operator at the master depresses the fourth push-button and thus energizes a relay PR over the line BO to operate a relay BR in the common equipment which completes the circuit of a relay I originally prepared by the pushbutton and prevents the operation of the PR relay in any subsequent calling station by the insertion of a resistor OR in the battery feed. Relay I operates a relay BS to switch on the anode supply for the amplifier through an intermediate relay AM and connects the transducer SI at a and b. The effective direction of the amplifier is controlled by the relay TR in response to the key TK at the master station. The amplifier includes a dummy load L which is inserted during calls to single substations and removed by relay Pa during group calls which are set up by depressing the fifth button of one of the switches PB. During such a call all the substations are connected by a relay Pa and the relay TR is disconnected. Loud-speaker masters such as LMI may call substations by throwing keys KSI, KSII to operate relays PR and BR as before and the line connection is completed over the cable ICm. If additional masters having no access to the common equipment are provided, communication to the substations may be established by acoustic coupling in the loud-speaker master. Operation of key KLII completes a circuit from source MTB to a buzzer relay RS at master station LMII and a three-line circuit having the battery MTB in the centre leg and including a microphone and a remote loudspeaker in each outer leg is set up when the called party responds to the call. A key HK is provided to switch from loud-speaker to hand-set working and the transformers MT introduce sufficient mismatch to reduce the level in the hand-set receiver. A busy tone signal source derived from the mains frequency or from a high frequency buzzer BE is included in the common equipment and a choke CE in the loud-speaker masters prevents busy tone from reaching the speech circuits.