Patent GB-598305-A (GB)
Title: Improvements in or relating to party-line intercommunication systems
IPC: H04M7/16
Filed: 1945-01-22
Abstract: 598,305. Party line telephone systems. ELECTRICAL COMPONENTS, Ltd., and SOMMER, W. Jan. 22, 1945, Nos. 1739 and 13839. [Class 40 (iv)] In a party-line telephone system facilities are provided for one-to-one calls and group calls as well as for all stations to hear public address transmissions. As shown in Fig. 4, each station is provided with a uniselector IU in the home position of which the receiver and transmitter circuits are completed over the banks IU1 and IU2 respectively. Carrier frequency from the oscillator OSC1 is modulated either by the microphone M1 or the dial DS1. To initiate a call, the subscriber throws the switch KA which connects the oscillator output to line, disconnects the H.T. supply from the speech receiver REC3 and disables the impulse receiver REC2, and dials the required number. The uniselectors of all stations other than that of the caller are stepped under the control of the receivers REC2 and depending upon the wiring of the uniselector banks the receivers and transmitters of one or more stations are connected up. In position 10 the receivers of all stations are connected up for the reception of the public address transmission. At the end of a call, the selectors are reset by the transmission of impulses by the calling party. In a modification, Fig. 6, the bank IU1 is in the receiver input circuit and the public address transmission is received over the contact c1 of relay C independently of the uniselector (the second wire of the receiver input circuit is not shown). A separate frequency is used for this transmission and the input circuits include filters. When the uniselector is stepped to a position characteristic of the station, C pulls up over bank IU3 and opens the receiver input circuit, while an alternative circuit is completed over bank IU1 for use in the ensuing call. Resetting of the selector at the end of the call reconnects the public address circuit. A further embodiment (Figs. 9, 10) provides simultaneous calls on two frequencies, public address reception on a third frequency and also allows an emergency call to be initiated on a fourth frequency by any station. The station shown is selected by two trains of impulses to which REC2 responds and repeats them to the driving magnets DM1, DM2 of the two uniselectors. Relay P responds to the first impulse and holds during the train, and is followed by IP which disconnects the release magnets 1ZM, 2ZM. It is assumed that the station number is 35, so that when IU is stepped to position 4, the relapse of P connects up relays R and S. The former operates to change over the impulsing circuit to DM2, but the latter being very slow to operate does not pull up at this time. In response to the second impulse train, 2U is stepped to position 6 where the release of P brings up Q which opens the input to REC2, extends the talking circuit and opens the circuit of S in time to prevent its operation. Two input circuits are associated with the transformers TR1, TR2 which are each tuned to one of the two frequencies used for normal calls. Assuming the incoming carrier frequency is that to which TR1 is tuned, the cold-cathode tube CC3 will strike followed by the operation of IF and, later, IE. Relay IF switches on the receiver REC3 and connects the secondary winding S2 thereto, while IE by a mechanical interlock moves the armature of 2E (Fig. 10) to the operated position and connects R3 across the tube CC4. Conversation may then take place, and if the called party wishes to speak, he throws the key IKA which disconnects TR1 from the line and connects it to the oscillator OSC1 for use as the frequency determining circuit, opens the receiving circuit and connects the oscillator output to line. The called party initiates homing of the switches by operating the push-buttons S1 and S2. If the first incoming impulse is not 3, IU restores to normal on the relapse of P and the second impulse train re-operates DM1. For second digits other than 1 or 3, IU is again restored on the relapse of P, while for 1 and 3, R comes up but with Q back, S initiates homing of 1U. If the first digit is 3, R comes up and the second digit steps DM2. Digit values other than 5, 7 or 8 result in the homing of 2U and 1U under control of 1P and T respectively. Initiation of a second call will cause LP5 to glow at all stations as well as CC4 at the stations partaking in the first call. In response to this indication, a busy party may change over to the second frequency by throwing key KC and then return to the first if required by throwing key KB. To initiate a call, the key 1KA or 2KA is operated depending on the frequency to be used. Besides the functions given above, the key also brings up Q which disconnects REC2 so that the impulses dialled are ineffective at the calling station. After dialling, the key is restored and the reply of the called party brings up 1E and IF or 2E and 2F. Public address transmissions are received over TR6 tuned to their carrier frequency, relay J pulling up to the frequency and switching through to the receiver. These transmissions may be interrupted by operating the key KF when a call is to be made, or by the operation of relay C on an incoming call. An emergency call incoming over transformer TR5 operates relay H which switches through to the receiver, disconnects the receiver for all other calls and opens the oscillator output circuit. Such a call is initiated by operation of key KE which connects the tuned circuit LC1 to the oscillator and completes the output circuit to line. The embodiment of Fig. 7 (not shown) includes automatic switching from receive to transmit controlled by speech or impulse modulation, while that of Fig. 8 (not shown) includes a relaxation oscillator which may be connected to the line over the uniselector bank and wiper to transmit characteristic tones when the station is called.