Patent GB-677894-A (GB)
Title: Improvements in or relating to intercommunication telephone systems
Inventor: ADLER ADOLF ERWIN
IPC: H04M9/00
Filed: 1949-03-01
Abstract: 677,894. Direct-call systems. MODERN TELEPHONES (GREAT BRITAIN), Ltd. Feb. 28, 1950 [March 1, 1949], No. 5588/49. Class 40 (iv). In an intercommunication system having talking and signalling batteries common to all stations and in which the stations have a test relay which is effective on calling to prepare the speech circuit and on being called to energize a calling device, break-in on an established call is prevented by the test relay at the calling station changing the potential applied to the calling line so that the test relay of any further station connected to the home line of either station is unable to operate. In Fig. 1, operation of a push button connects the substation shown to the home line of another substation, energizing a relay S over its high resistance winding a20 at the calling station and a buzzer BZ14 at the called station. The relay S in operating connects the speaking equipment to the selected line via a condenser C13 and connects a low resistance coil b21 in parallel with the high resistance coil a20 to shunt any further S relays which may be connected by further calling substations. The substation incorporates a Wheatstone bridge coupling arrangement for the microphone and receiver as described in Specification 650,070, but an alternative arrangement in which the microphone and receiver are in series is described with reference to Fig. 2 (not shown). In another system, Fig. 3, the test relay R has a single winding and in addition loud-speaker master stations, Fig. 6, are connected over lines M32. A change-over switch K39 is provided at the substation to switch the handset HS from the Wheatstone bridge arrangement BC40 used for inter-substation communication, to the line M32 and a calling lamp ML44 operating from the master battery MTB46 is provided to indicate to an engaged substation that a master station is calling. In inter-substation communication the test relays R are energized at both stations, the buzzer BZ53 and a series resistor are shunted at the calling station to prevent the calling station being itself called and a local buzzer circuit is closed at the called station. A condenser CR51 feeds back a ringing tone to the calling station until the buzzer circuit is broken by the handset contacts which also release the local relay R. The resistances N47 and Q50 in series are sufficient to prevent any further substation energizing its test relay over the home line of the calling party, even after the resistor Q50 is again shunted when the test relay at the called station is released by the call being answered and the short circuiting of resistor P49 at the calling station will protect the home line of the called station. In inter-substation communication a push-button may be inserted across the straps Y to start a call after selection has been effected by one of the push-buttons U40, V41 ..., and in addition a lamp BL52 may be provided to show that the local test relay has not operated, i.e. the called substation is engaged. Calling lamps SL64 individual to each substation are provided at master stations together with keys MK65 for calling or replying to each substation. The master station test relay MR short-circuits resistor MP66 and applies a negative potential to the home line of the connected substation to prevent break-in. In a modified system, Fig. 4 (not shown), mechanically interlocked series - connected change-over switches are employed for line selection and a common ringing bar is incorporated. The switches are arranged to effect the selection before the ringing contacts are closed and to maintain the line connection when the push-button is released from its calling position. The circuit may include rotary selectors in the outgoing lines from the push-buttons. Specification 650,070 is referred to.