Patent GB-680765-A (GB)
Title: Improvements in or relating to intercommunication telephone systems
Inventor: IVENS EDGAR
IPC: H04M9/00
Filed: 1949-09-16
Abstract: 680,765. Direct call systems; loudspeaker systems. IVENS, E., and MODERN TELEPHONES (GREAT BRITAIN), Ltd. Sept. 15, 1950 [Sept. 16, 1949], No. 23972/49. Class 40 (iv). In a loud-speaker intercommunication system comprising master stations, an amplifier for each master station, and substations, the transducers may be used both as microphones and loud-speakers and the effective direction of transmission is relay controlled in response to a switch at the substation or in the case of inter-master communication by a switch at the junior master. In a system, Fig. 1, the speech circuit between the master station and any of the substations I, II, III is completed over lines a, b by the appropriate key K and a key TL in the substation controls a reversal relay TR over a line c. The key TL may also be used to call the master over lines band c to light a calling lamp LI, LII or LIII and energize a buzzer BZ over a relay RS. Master break-in is provided at key IN and a key LT is also provided for local control of the reversal relay. The substations are voice-called after the master has thrown the appropriate key K and a resistance R may be inserted by automatic means, e.g. a gravity switch, when the listener is close at hand. In a further system, Figs. 2a, 2b, a common earth CH is used to complete the talking circuit, volume controls VC are provided, and a relay TRB, having contacts in the amplifier input circuit and shunted by a condenser C, delays the closing of the input circuit to reduce " click " in the loud-speakers. A minor substation SII not having a volume control is connected to the amplifier in the station, Fig. 2a, over a strap between lines IIa, IIb, but in the junior-master shown in Fig. 2b the strap is omitted and in consequence the minor station is only able to receive messages. In order to avoid interference the engagement of a substation by one of the masters is indicated to the other master by lamps EL. A crystal microphone CR is provided in a further system, Fig. 3, for inter-master communication and in such communication each master uses its own amplifier for outgoing speech, which is fed to a distant loud-speaker over the home lines of the distant station. In order to reduce acoustic feedback, the calling master must remove his transducer from its rest so that resistors R1, R2 are inserted by a gravity switch SWA. The batteries BB and AB are connected to the amplifier via key KR which is normally locked as shown and is mechanically released by the selector keys KM, KI, KII. In communication between a master MI and a substation I, switch TL controls the reversal relay TR over the common earth and the home lines of the substation in parallel, the circuit being completed at the master station by a line from the centre tap of resistors RM connected across the line. The paralleled substation lines are used in a similar manner for completing the circuits of the calling lamps LI, LII. In a transducer set, Figs. 4, 5 (not shown), the key TL is a pushbutton and a spring-mounted pin projecting from the base operates a switch to effect a reduction of volume when the set is lifted.