Patent GB-1187958-A (GB)
Title: Improvements in or relating to Telecommunications Systems.
IPC: H04M11/06
Filed: 1967-06-28
Abstract: 1,187,958. Multiplex systems; telegraphy; data systems. SIEMENS A.G. 28 June, 1967 [29 June, 1966], No. 29804/67. Headings H4K, H4P and H4R. Telegraph signals are transmitted over a frequency band within the speech frequency band over telephone lines carrying normal speech traffic. A particular part of the telegraph channel is used for transmission in each direction alternately, and a further part of the telegraph channel is used to transmit control information in the reverse direction. As described the telegraph channel occupies the band from 1000 to 1500 cps., information being signalled by frequency shift between 1225 and 132C cps. and the control signal being either 1045 or 1095 cps., dependent on the control condition. Fig. 3 shows an arrangement in which a switching arrangement Fg has connections via a two wire line a to a teleprinter, via a four wire line b to a teleprinter exchange, and via lines C1 and C2 and the transmitting and receiving circuits to a hybrid W and hence to the line F1. During rest period when no signalling is taking place a negative signal is provided by the switching circuit Fg on line C1, and this signal ensures that oscillator G emits the lower information signal frequency which, due to the fact that no output is available from G1 and therefore contacts U1 to U5 are in the condition shown, is blocked by filter SKF which passes only the control signal frequencies. In order to make a connection a positive signal is put on line C1 by switching circuit Fg and this signal, via delay circuit # changes generator G to its upper information signal frequency. The positive signal on line Cl, via delay circuits #+ and #-, actuate "AND" gate Gland operate switches U1 to U5 so that the information signal frequency passes via filter NaF and the send amplifier SV to the hybrid W and outgoing telephone line F1. At the far end of the line F1 the second station receives the incoming upper information signal frequency which passes via the unoperated contacts U4, the filter NaF, and contact U3, to the discriminator and decoder E. The level monitoring device P produces an output which is applied, together with the inverted negative output of gate G1 to operate a gate G2 so that it produces a positive potential which operates a relay R in generator G to tune the generator to the control signal band, while the positive output of G2 via the inverter J inhibits gate G1 to maintain the contacts U1 to U5 unoperated so that the control signal output of generator G passes to line F1 via the filter SKF &c., and the channel is maintained for information reception. The reception of the upper information frequency at E causes positive potential on line C2, which via switch unit Fg, makes the teleprinter ready for operation. When the teleprinter is ready for signal reception positive potential is applied by switch unit Fg to line C1 which via delay unit #switches generator G to the upper control signal frequency which signal is fed, via filter SKF, to the line F1. At the first station the incoming upper control frequency signal from the second station is received via the operated contacts U3 and U4 and filter SKF, and via the discriminator E operates the level monitoring device P and supplies positive potential on line C2 to the switch unit Fg to cause the teleprinter at the calling station to be made ready for operation. 500 M/secs. after the application of positive potential on line C1 the output of delay circuit #+ goes negative and inhibits gate G1 so that contacts U1 to U5 restore to the position shown, i.e. ready to receive information signals, so that both stations are now ready to receive teleprinter transmissions. Teleprinter transmission causes a negative start pulse on line C1 which gives immediate positive outputs from the delay units #+ and #- so that "AND" gate G1 produces an output that switches contacts U1 to U5 for information band transmission. The negative signal is applied via delay unit # to generator G so that it emits the lower information frequency signal as a start signal to the receiving teleprinter. Telegraph transmission may then continue for as long as desired. If during transmission a marking potential exists for longer than 500 M/sec. or a space for longer than 1 sec. the delay elements #+ and T- cause the gate G1 to be disabled and the transmitting station to revert to a receiving condition so that information transmission can be continued in the reverse direction. In order to break down the connection a negative potential is placed on the line C1 by a stop key in the switch unit Fg, which potential on reception at the receiver switches off the teleprinter until a further start signal is received. Switching off the teleprinter causes a negative potential on line C1 and the appropriate control frequency signal to be emitted which, on reception at the first station, causes that teleprinter to be switched off, leaving the whole system in the rest condition.