Patent GB-871583-A (GB)
Abstract: 871,583. Direct-call systems. ASSOCIATED ELECTRICAL INDUSTRIES Ltd., and POSTMASTER-GENERAL. June 16, 1959 [July 11, 1958], No. 22397/58. Class 40 (4). A transistor amplifier at a substation is powered over the speech wires by central battery reversal of which switches the amplifier from a receive to transmit condition. The sub-station is shown in Fig. 1 in which a transistor amplifier TR 1 feeds the push-pull pair TR2, TR3, working in class A. A transmitreceive transducer TX is connected between transformers T1 and S4 to receive or transmit according to whether relay RL is energized. Central battery over line Lla, L1b provides constant polarity bias for the transistors over a bridge RF and normally L1a is negative and L1b is positive so that relay RL is unenergized and TX is connected to receive. If the sub-station wants to transmit the contacts k1, k2 are made so that when the polarity of central battery is reversed relay RL energizes and a path is provided over bridge RF to supply the amplifier. The contacts k1, k2 are broken if the sub-station does not want to be overheard. Fig. 2 shows a master station connected to sub-stations over lines L1a, L1b to Lna, Lnb, each with an individual key, with contacts K1a,K1b to Kna, Knb. Central battery is connected to leads Nm, Pm, negative and positive respectively. To call a sub-station, say that of Fig. 1, contacts K1a, K1b are made and a ringing key with contacts kr1 to kr3 is thrown to connect the output of amplifier TRB to its input thus providing a ringing tone oscillator with resistor Ro shunted by capacitor Co in the feedback loop. Operation of contacts kr1 operates relay RLA to put negative battery on lead L1a and positive on lead L1b causing amplifier, now oscillator, TRB to be connected to transmit with transmit-receive transducer TX1 disconnected. At the sub-station the polarity of battery on the line is not such as operates relay RL so amplifier TRA is connected to receive the ringing tone and an answer may be given by operating contacts k1, k2 and waiting for the master station to release the ringing key. With contacts kr1 to kr3 back, relay RLA releases connecting amplifier TRB to transmit and reversing current in the line to operate relay RL at the sub-station to set amplifier TRA to receive. Conversation proceeds under control of a transmit-receive switch with contacts sw1 to sw3 at the master station. To call the master station the substation of Fig. 2 operates contact k1, k2 to complete a loop circuit to light lamp L1 and energize relay RLB at the master station where a cancellation key with contacts Kx1, Kr2 is normally thrown and has an interlock which releases any thrown line keys. Relay RLB connects amplifier TRB to oscillate without the components Ro, Co in the feedback loop, thus providing a tone which differs from ringing tone to the sub-stations, and connects transducer TX1 in series with resistor. Rt. To reply, the central station throws the line key of the lighted lamp L1 automatically releasing the contacts Kx1, Kx2, by means of the interlock, and this releases relay RLB to stop ringing. The call may now be acknowledged by throwing the transmit receive key. If a call is already being made to the master station relay RLB will be disconnected by a sub-station call which will light its associated lamp L1 to Ln. A bell or buzzer on terminals tt may augment ringing at the master station. Speech is decoupled from the amplifier supply at the master station by inductors CHn, CHp, non-linear resistors Rx being used as shunts to discharge the inductors when battery is reversed. Speech is decoupled at the sub-station by a capacitor C1 across the amplifier supply lines.