Patent GB-693165-A (GB)
Title: Improvements in or relating to electric signalling systems
Inventor: DUNN RONALD HARRIS
IPC: H04J3/00
Filed: 1949-09-27
Abstract: 693,165. Electric selective signalling systems. STANDARD TELEPHONES & CABLES, Ltd. Sept. 22, 1950 [Sept. 27, 1949], No. 24812/49. Class 40 (i) [Also. in Group XL(b)] In an electric signalling system in which a code made up of a series of signal elements, such as the signal for operating a remote indication system as described in Specification 686,829 is transmitted by modulation of a carrier signal, synchronous operation of the translating equipment at the receiving terminal and the equipment timing the signal elements at the transmitting terminal is effected by employing the carrier frequency to govern the timing of both transmitting and receiving equipment. The invention is described for a frequency shift system employing a carrier of 2000 c/s. Frequency dividing circuits at the transmitter derive 50 c/s pulses from the transmitter carrier oscillator for controlling the code sender, and similar circuits at the receiver derive 50 c/s pulses from the received carrier for timing the decoder. A phase shift is produced in the receiver 50 c/s pulses to ensure that the decoder scrutinizes the signal elements in the middle of a signal element period. The change of frequency between the alternative transmitter frequencies is small and gives rise to a negligible change in the time occupied by a signal element. There is no cumulative mis-timing as between transmitter and receiver since the timing signals at each end of the channel are varied in step with one another. Transmitter, Figs. 4 and 5. The carrier oscillator 1, Fig. 4, is shifted between its two frequencies by relay TRK which is operated under the control of code sender 75-79 in accordance with the code 'set upon a series of contacts 86 ... 87. Connected to the signalling line 2 is an amplifier 34 at the output of which appear amplified sine waves at the instantaneous carrier frequency. The sine waves are squared in stage 35, differentiated by network 39, 40 and limited to positive excursions by rectifier 41. These positive pulses, one for each cycle of carrier frequency, are then passed to phase splitting stage 42, 43 to derive a negative and a positive pulse train at the outputs P- and P+ respectively of cathode followers 46 and 49. The two trains are applied to a 40:1 dividing stage, Fig. 5, which provides from a 2000 c/s carrier a final impulse frequency of 50 per second. The positive pulses (P+) trigger a four-stage counting ring 50-53 comprised of gas discharge tube circuits units 1, Fig. 2 (not shown) and at every fourth pulse open a gate 54 to allow a negative pulse (P-) to trigger a multi-gap discharge tube 60 of the type described in Specification 692,415, [Group XL(a)]. The tube has ten gaps in a ring which fire in succession, thus, at every forty cycles of the carrier gap 63 fires and a positive pulse appears across circuit 64, 65. These pulses are amplified without phase change in stage 71 and appear at the output 1<SP>1</SP> of cathode-follower 74 limited between earth and +SV3 by rectifiers 72 73. The code sender comprises a ring 75-79 of discharge tube circuits similar to those employed in the divider, and the 50 c/s pulses from terminal I' are applied to trigger the circuits in succession. The positive potentials which appear at the cathode circuits C are then applied to the mark (M) or space (S) line in accordance with the setting of code contacts 86 ... 87 causing the relay TRK to take up the condition of each signal element in turn with a timing determined by the 50 c/s pulses. The first circuit 75 in the ring is connected permanently to the space line to provide a start signal and the end two 78, 79 are connected permanently to the mark line to provide a stop signal. Receiver, Figs. 6 and 7. The signals on line 2 are applied to receiving detector 3 and operate receiving relays TRA, TRB in accordance with the code elements, the elements being distributed to individual utilization devices under the control of decoder 99-103. To control the timing of the decoder 50 c/s pulses are derived from the received carrier by amplifier 34', pulse shaper 35' and frequency divider circuits 50'-53' and 60' generally similar to those employed in the transmitter. The pulse output from the differentiator and limiter circuit 39<SP>1</SP>, 40' 41<SP>1</SP> however is connected to the phase splitter 42', 43' via a start-stop circuit containing a gate 89, 90 which is normally rendered non-transmissive by discharge tube 91. This tube in conducting biases tube 90 positive and cuts off tube 89. Upon receipt of a start signal relay TRA moves to space and contact tra breaks the anode circuit of tube 91 allowing the gate to transmit pulses to the phase splitter. The frequency divider rests with circuit 50' operated and the first cathode 63<SP>1</SP> of tube 60' fired. The output connection for the 50 c/s pulses is taken from the sixth cathode across circuit 95, 96 and it takes therefore 20 cycles of the carrier after receipt of the start signal before the first timing impulse appears on terminal I<SP>1</SP>. Thus the receiver 50 c/s pulses lag 20 carrier cycles behind those at the transmitter and subject to line distortion occur half way through the received signal elements. The 50 c/s pulses are applied to trigger in succession the stages 99-103 of the decoder counting ring (similar to that in the code-sender) and are also applied through contacts trb of receiving relay TRB to cathode-followers 97, 98. By this means decoding units 105-108 units 2, Fig. 3 (not shown) are rendered responsive in succession by priming potentials applied at terminals B from the counting ring, each unit triggering to either mark or space from the positive pulse which appears at terminal PS or PM in dependence on the setting of contact trb. The units control utilization outlets which may be the indicators, lamps or the like in the remote indication system. At the end of a code when the stop signal (mark) is received unit 103 in the decoder counting ring is operated and at terminal ECC applies positive bias to tube 91 in the start-stop circuit. The potential is insufficient to fire the tube and pulses continue to pass to the divider until tube 60' fires at its first cathode 63'. At this point the combination of biases on terminals ECC-ECF cause tube 91 to fire from the next positive pulse from 49'. The flow of pulses to the divider ceases and the unit rests awaiting the next start signal. The transmitter coder and receiver decoder may be replaced by circuit chains of the " tree " type described in Specification 654,098, [Group XIX]. Specification 692,411, [Group XL (b)], also is referred to in the Provisional Specification.