Patent GB-381831-A (GB)
Title: Improvements in or relating to electrical communication systems
IPC: H04B3/10
Filed: 1932-02-27
Abstract: 381,831. Two - way transmission. WESTERN ELECTRIC CO., Ltd., Bush House, Aldwych, London.-(Bell Telephone Laboratories, Inc. ; 463, West Street, New York, U.S.A.) Feb. 27, 1932, No. 5937. [Class 40 (iv).] In a system for transatlantic cable telephony involving a number of repeater stations between city terminals in London and New York and the respective cable terminals, each city terminal is provided with a voice-operated gain adjusting device or VOGAD which maintains a constant volume for the transmitted speech in spite of varying line conditions, a voice-operated anti-singing device, or VODAS, which normally disables the transmitting or " go " path and comprises means for giving control to speech arriving from the cable whether or not there is speech on the " go " circuit and an arrangement for adjusting the received volume in accordance with a number of variables including the sensitivity of the voice-operated relays in the VODAS, the kind of telephone line connected up, noise conditions &c. The VOGAD and the VODAS are both able to distinguish between steady noise and speech. Carrier links are employed between the city and cable terminals. Signalling is effected by 1000-cycle current interrupted at 20 cycles. Artificial noise may be provided. The circuits shown can be used in a system in which the main cable is replaced by a radio link but this does not form part of the invention. General lay out, Fig. 1. The terminal station at New York is connected over a repeatered line to Bangor (Maine). Speech over this circuit is then repeated on a carrier channel which with other carrier channels, ordinary telephony and low frequency telegraphy is taken over a 2-wire open line to Sydney Mines and then over submarine cable to the main cable terminal at New Perlican with a repeater point at Port aux Basques. Speech from New York is then demodulated and passes to the cable which may also carry low frequency telegraphy and terminates at Termoncurragh. Between here and Stranraer one direction of speech is carried as ordinary telephony and the other as a carrier channel. This cable is used also for carrier telegraphy. From here to London transmission takes place over ordinary 4-wire circuits. VOGAD, Fig. 2. The gain of amplifiers 12, 13 is determined by the voltage on condenser 15. Speech arriving from the telephone line energizes relays 17, 18 to discharge 15 and so increase the gain until the amplified speech is able to energize 23 which opens the discharge circuit. Further increase in the level reduces the impedance of the tube 24, increasing the charge on 15 and reducing the gain. 23 on operating biases its valve in favour of operation to prevent too frequent changes. 17 is guarded from operation by noise by providing it with opposing windings fed from two paths one of which, owing to the presence of a lowpass filter favours the syllables of speech. VODAS, Fig. 3. A portion of the speech in the " go " circuit is selected by a filter 33 which passes the most energetic speech frequencies (thus discriminating against noise) to a full wave rectifier 34 controlling directly a relay 38 and through a transformer 49 and syllable frequency filter 35, oppositely connected polarized relays 36, 37 which pull up at the beginning and end respectively of each syllable. 38 is normally energized and shunts 39 .. 44, this shunt being removed when 36 or 37 operates. On operation, 40 removes the anti-singing shunt from the " go " circuit, 41, 44 control a secrecy circuit (not shown) in the case of wireless transmission and 43 disables the " return " circuit. When speech ceases. condenser 45 charges through hangover windings on 40, 41, 42 and when 42 falls condenser 46 charges through hangover windings on 43, 44. Abnormally strong signals release 38 which produces the same effect as the energization of 36, 37. 39 on operation energizes 53 which initiates the release of 54. If 53 remains energized for (say) 10 seconds, as in the case of abnormal noise, 54 is released and energizes 55 which locks over an alarm circuit and energizes 56 to cut a 9 db. pad into the path of the detector 34 to reduce its sensitivity and 57 to make a corresponding adjustment in the pilot circuit controlling the received volume. Strong speech arriving in the " return " path energizes the breaking-in relay 76, the sensitivity of which is manually adjusted to make it inoperable by the strongest interference, to energize 77, 78, 79 which release 39 .. 44 if necessary and shunt their hangover windings. A further portion of the speech passes to a detector 84 to control relays 85, 86, which on operating energize 87, 88, 52, 91. 52 disconnects 39 . . 44, 87 shunts 36, 37 and disconnects filter 35 to prevent energy stored in it from operating the transmitting relays when 87 falls, and 88 disables the VOGAD. 91 cuts off 92 but not long enough, in normal speech, for its release. In the case of abnormal noise however 92 is released energizing 93 which locks to an alarm circuit, 94 which cuts in a 9 db. pad to reduce the sensitivity of detector 84, and 95 which makes a corresponding adjustment of the pilot circuit to reduce the received volume. When speech ceases, 85, 86. and 76 fall at once, 77, 78, 79 after a hangover fixed by condenser 98 and 52, 87, 88 after a hangover fixed by condenser 101. Controlling received volume and sensitivity of VODAS. 6000 cycle current from 27 after amplification in the VOGAD passes through a pilot circuit comprising a number of adjustable loss networks to a detector 124 which controls relays 125, 126 so that when the loss in the pilot circuit is high 125 falls and when it is low 126 pulls up. When 126 pulls up R is energized and P, Q, S interact to deliver impulses to the energizing windings of the relays A .. O which operate like a counting chain, locking up mechanically and cutting loss in 3 db. steps into the "return" circuit 20 at 110 and into the pilot circuit at 123, until 126 is released. When 125 falls, P .. S operate as before but in this case deliver impulses in the reverse order to the releasing windings of the relays A .. O until 125 pulls up again. Whenever the sensitivity of the " go " side of the VODAS is adjusted either by the manually adjustable pad 103 or by the pad cut in by relay 56 under abnormal noise conditions an equal and similar variation is made in the pilot circuit at 106, 118. When the sensitivity of the " return side is adjusted at 82 manually or by relay 94 under conditions of abnormal noise or by relay 146 when the connection involves a radio link, equal and opposite adjustments are made at 107, 95, and 145 in the pilot channel. Further adjustments of the pilot circuit loss are made at 113 corresponding to the manual adjustment 109 of the received volume and at 120 by the technical operator independently of any other adjustment. The pad 121 is cut out when the circuit is used with a long two-wire telephone line, the pad 138 in the "return" circuit being cut out at the same time. Connection to telephone lines. A short two-wire line is connected up by plugging into jack 131 with a plug having an ungrounded sleeve. In the case of a long two-wire line the plug sleeve is grounded and sleeve relays 140 .. 143 are operated to cut out loss pads from the " go," " return," and pilot circuits and increase the hangover of relays 87, 88, 52. Where the toll line involves a radio link, 145, 146 are energized also to reduce the receiving sensitivity and increase the loss in the pilot circuit. A four-wire line is connected by plugging into both jacks. Relays 140 .. 143. 134, 149, 150 pull up. As a result the " go " and " return " circuits are extended separately and the pad 138 is cut out but no adjustment is made of the receiving sensitivity or pilot circuit loss or of the hangover time of the receiving relays. Carrier terminal, Figs. 4 and 5. Speech arriving in the " go " circuit 10 passes through a volume limiting amplifier 160 the gain of which is a function of its output but which is designed so that variations take place slowly and a peak voltage limiter 161 comprising a valve with a high resistance in its grid circuit to a modulator 163. To ensure complete suppression of the carrier, the plates of the modulator are fed through a sliding contact on a resistance. This resistance in conjunction with series resistances also compensates for the variations in impedance at different frequencies of the networks 174, 175. One side-band is selected for transmission by the filter 174 which is assisted by a network 175 for suppressing frequencies just outside the lower limit of the pass band of the main filter. These networks may have rounded characteristics, compensation being provided by distorting networks 162 in the low frequency circuits. The side-band together with sidebands from other channels is amplified at 177 and 178 and passed to the line 165. Carrier speech arriving from line 165 is directed by filter 180 through a transformer for stopping longitudinal currents, an attenuation equalizer and a volume regulator 183 controlled manually in accordance with the readings of a pilot indicator or automatically by the pilot current to the filters for separating the channels and is then demodulated. Filters 185 have cooperating suppression networks 186. The distortion introduced by these networks is corrected in networks 189. Carrier repeater, Fig. 6. The " go " carrier channels are separated and passed through an attenuation equalizer and a volume regulator adjustable by hand or automatically to an amplifier resembling those described in Specifications 317,005, [Class 40 (iv), Telephones &c.], and 371,887, [Group XL]. This amplifier has an odd number of stages and comprises a feedback connection from the last to the, first whereby the overall gain is reduced but the ratio of signal to distortion is improved. Two or more amplifiers may be worked in parallel provided they are adjust