Patent GB-646500-A (GB)
Title: Improvements in carrier telephone transmission systems
IPC: H04Q1/442
Filed: 1947-04-17
Abstract: 646,500. Carrier transmission. PUNCH ENGINEERING PTY., Ltd. April 17, 1947, Nos. 10232 and 10233. Convention dates, May 6, 1946, and Aug. 19, 1946. [Class 40 (iv)] In an exchange system employing a carrier link, signals normally transmitted in a telephone system as steady state direct current signals are conveyed over the carrier link by steady state changes of level of the carrier waves transmitted over the link. A call or supervisory signal is transmitted by a rise in carrier level transmitted in the direction of the signal and a forward or backward clear signal by a fall in level. In one embodiment the higher level of carrier is that considered suitable to demodulate the transmitted side-bands at the receiving end, and the lower level is substantially zero. In order to transmit busy and ring-back tones and to permit operator's conversation in the backward direction before the carrier level is raised to convey a supervisory signal, an oscillator is provided at the receiving end to supply carrier current when required. In another embodiment, the lower level of carrier transmitted in the backward direction is the maximum value which may be used without either actuating or preventing the release of the signalling device, no local oscillator being then required at the receiving end. First embodiment (Fig. 1). The channel terminates in jack 1 and the go path leads over transformer 2, low-pass filter 3 and hybrid 4 to the modulator comprising transformers 5 and 6 and rectifiers 7. A volume limiter consisting of transformer 8, rectifiers 9 and potentiometer 10 is provided. Carrier from generator 11 is supplied to the modulator and also to the outgoing line via an adjustable resistor 12 and a front contact of relay 13 which comes up when the operator plugs in to the jack 1. The receiving equipment comprises band-pass filter 15, potentiometer 16, and detector 17. The voice-frequency and direct current voltages produced across resistor 19 and condenser 20 by an incoming signal cause valve 23 to conduct and anode relay 24 pulls up. Resistor 25 is short-circuited, operating a marginal relay (not shown) in the sleeve circuit to control the supervisory lamp. When the transmission of carrier current is interrupted the D.C. voltage across resistor 19 is not sufficient to overcome the normal bias from potentiometer 22 and relay 24 releases. If the far end of the link is at an automatic exchange the operation of the relay corresponding to 24 causes seizure of the first group selector and when the operator dials relay 13 responds and repeats the impulses to the relay corresponding to relay 24. Local carrier from generator 26 is used to de-modulate signals received prior to the carrier being transmitted in the backward direction when the call is answered. This local carrier is at a low level and as a result valve 23 operates at a less steep part of its characteristic and produces less gain than when carrier is transmitted in the backward direction. To provide constant overall gain relay 24 may introduce a pad (Fig. 2) between points a, b, and c, d (Fig. 1), when the carrier is transmitted. Second embodiment (Fig. 4). This embodiment is used between two automatic exchanges. Relay 43a at the outgoing end X receives signals from exchange 31 and its operation removes the short-circuit from the go-path 55, thus permitting transmission of carrier current from generator 46 at full strength whenever the channel is seized. A similar relay 43b at the incoming end Y operates when the called party answers and short-circuits resistors 47, 48 which are included in the carrier supply path up to that time. Operation of 43b also removes the pad 56 at the incoming end Y. The full strength carrier operates relay 54a which inserts pad 57 at the outgoing end X to maintain the overall gain constant, and reverses the polarity over the line to exchange 31. Relay 43a repeats dialled impulses as interruptions of carrier over the go-path, the relapses of 54b repeating the impulses over the loop to exchange 32.