Patent GB-338973-A (GB)
Title: Improvements in or relating to electric telephone and like transmission systems
IPC: H04B3/20
Filed: 1929-08-29
Abstract: 338,973. Electrical Research Products, Inc., (Assignees of Mathes, R. C., and Horton, A. W.). June 11, 1929, [Convention date]. Repeating and amplifying systems. -In a system including a line of high attenuation, such as a submarine cable, the input circuit is provided with volume control devices and wave-shaping networks which reduce the input currents to currents of substantially the same volume level from instant to instant and of substantially the same energy-frequency level, except for such departures as may be required to compensate for noise or distortion in the system. Fig. 1 shows a terminal repeater system connecting the submarine cable CL to the land line LL, the transmitting path TC being normally disabled and the receiving path RC normally operative. The apparatus shown may be located at a single geographical point, or at different points as may be most advantageous in practice. Control of operative condition. This forms the basis of another Specification. Speech currents arriving over the land line LL pass through the path TC, a portion being diverted into the circuit 18 to release a normally energized relay 20, which disconnects a sluggish relay 21. During the release period of the latter, relay 23 energizes and connects up a generator 25 to send out a courier impulse of frequency f, over the cable CL through filter 26, amplifier 27, hybrid transformer 28 and filter 29. Relav 20 also closes a circuit for a relay 35 which is rendered slowoperating by a parallel condenser 32 and which connects up relays 17, 40, to complete the transmitting path TC and disable the receiving path RC, and relay 40<1> to prevent reception of a courier impulse from the cable. A sluggish relay 41 also energizes and short-circuits the output of the amplifier 53 in the path RC in order to prevent echo currents when relay 40 subsequently releases. A delay circuit 14 in the path TC delays the speech currents preferably for a sufficient time to allow the necessary circuit changes to take place at the distant end of the cable. Assuming speech starts at the distant end of the cable, the incoming courier impulse of frequency f2 passes through filter 29, hybrid transformer 28, amplifier 42, and filter 43 to an amplifier rectifier 44 to energize a relay 37, which connects up a relay 16 to further disable the transmitting circuit TC and opens a point in the circuit of relays 17, 40, 40<1>. A relay 50 also energizes, so that a part of the incoming speech currents over the receiving path RC is diverted through filter 51 to release a normally-energized relay 38 over the circuit 68 to maintain the circuit changes effected by the courier impulse after the sluggish relay 37 falls back. The control circuits 18, 68 include noise protection devices 19, 52. Transmission conditions. The filter circuit 9 passes the band of frequencies 300-3000. The wave-shaping network 11 is designed to bring the component frequencies more nearly to the same volume level, but at the same time to ensure that the level is sufficiently above that of the noise disturbances in the cable. The network. 59 is designed to produce the reverse effect. The volume control device 12 and the compensating control device 57 are such that the volume of the emitted currents is maintained substantially constant from instant to instant for varying levels of input volume, and are inter-related to keep the gain round the paths TC, RC substantially constant. The volume compressor 13 is designed to reduce the volume range of the different fre. quencies. An equalizer 111 may be provided in order to emphasize the higher frequencies, which are subject to greater attenuation in the cable. The filter 54 excludes the courier frequencies. The volume expander 56 is designed to reverse the effect given by the compressor 13, and preferably in front of it are arranged the attenuation equalizer 61, which corrects for the greater attenuation of the higher frequencies, and the corrective expander 55, which corrects for a non-linear volume compression effect in the cable. An auxiliary control 65 may be placed on the expander 55 to make the corrective expansion a function of the frequency as well as of the volume level. A delay circuit 59 is provided in the receiving path. Specifications 184,774, [Class 36, Electricity, Conducting &c.]; 202,284, 255,221 and 323,222 [Class 82 (i) Metals, Extracting, &c.], are referred to.