Patent GB-780947-A (GB)
Title: Self-correcting pulse-code-communication system
IPC: H04L1/00
Filed: 1954-03-23
Abstract: 780,947. Code telegraphy. HAZELTINE CORPORATION. March 23, 1954 [March 30, 1953], No. 8412/54. Class 40 (3). A pulse - code communication system in which an error in a group of code combinations of pulses representing characters is automatically corrected, comprises a transmitting arrangement in which a number of character code combinations are stored for the purpose of deriving a check code combination of elements uniquely representative of predetermined digital relations of the elements in the character groups which are transmitted and followed by the check group, and a receiving arrangement in which the received pulse groups are stored and a check group derived from the character groups as received is compared with the check group as received to enable a faulty code element to be corrected before the character groups of pulses are applied to recording apparatus, such as a teleprinter. In the system described five signals each comprising five code elements are stored to derive a five-unit check signal. Transmitting-arrangements.-At the start element of a code combination from the transmitter 50, winding 63a is de-energized and at closed contact 63b a pulse generator 64 is triggered into operation for the period of a signal combination and energizes a winding 71a which at contact 71b brings into operation a pulse generator 65 and at contact 71c connects the output of the transmitter 50 via a normally closed contact 52b to a shifting register 51 having twenty-five storage circuits numbered 30 ... 3. The generator 65 produces sampling pulses operating winding 52a so that contact 52b is intermittently opened, and closed at approximately the middle of the pulses produced by the transmitter 50 and also produces pulses applied to step forward in sequence the pulses after they have been registered in the circuit member 30. After five more combinations have been stored on the shifting register 51 the impulses are transferred simultaneously to a message digit register by the action of a keying pulse generator 67 operated when 25 pulses have been counted by the counter 66. The pulses set up in the register 53 are also applied to a check-digit computer 55 comprising sensing members 1a . . . 16a set into operation by a driving mechanism controlled by the generator 67. Each of the members 1a . . . 16a is arranged to examine fourteen of the code pulses stored in the register 53, the member 16a, for example, examining pulses 17 ... 30, as indicated by the table shown in Fig. 3. The remaining members 8a ... 1a examine other mutually different sets of pulses as displayed by the table and the arrangement is such that a particular pulse is examined simultaneously by two or more of the members 1a . . . 16a-pulse 7, for example, being examined by members 1a 2a, 4a, and pulse 30 by members 2a, 4a, 8a, 16a. Further, a different combination of the members 1a ... 16a is associated with each of the pulse elements examined. The pulses detected by the members 1a . . . 16a are passed to odd-even pulse counters 1b . . . 16b providing a positive or zero output in the case of an odd or even number of pulses in the fourteen elements examined. The check elements or pulses are applied to storage devices 1, 2, 4, 8, 16 of a message digit and check digit register to which the pulses from the register 53 have already been applied. The 25-message elements together with the 5 check elements are passed via distributer 58 set into operation by a driving mechanism 59 controlled by the generator 67. The sequence of elements is prefixed by a two-fold synchronizing signal producing marking and spacing elements of unit length apart from a marking impulse of 1¢ units from segment 69 and a spacing impulse of 2¢ units from an earthed segment 70. The signals are sent from transmitter 61 as frequency shift modulations. A reset pulse generator 68 provides pulses at appropriate times to restore the registers 53, 54 and computer 55 to reference conditions in preparation for the next set of message signals. Detection of synchronizing signal.-The circuit arrangement is as described in Specification 780,948, and the output of the radio receiver 81 is initially applied to the device 96 in which the signals are recorded on a magnetic tape 122 fed past a recording head 121 by rollers 123, 124. The impulses are picked up by coils 126-131, 138-142 spaced according to the leading and trailing edges of the pulse pattern, Fig. 7, and associated with generators 150 ... 160 interconnected with three groups of conducting rectifiers 132 ... 137, 143 ... 148 and 161 ... 166 connected to points 180, 149 and 167, resistors 168, 169, 170 and via rectifiers 171, 172, 173 to a winding 174. The coils 127, 129 . . . associated with the trailing edges of the pulses are reversed so that the generators 150 ... 160 all produce positive impulses. If all the rectifiers of a group 132 ... 137, 143 ... 148, 161 ... 166 are made non-conducting, current flows through one or more of the resistors 168, 169, 170 so that winding 174 is energized to close contact 175 and operate the pulse generators 97. The three sets of rectifiers are provided so that the winding 174 is operated if the first or second part of the synchronizing signal is received undisturbed or if the mutilation is limited to the beginning and end of the first part and the beginning of the second part of the composite signal. Receiving - arrangements.-When the synchronizing signal has been received the pulse generators 97 are triggered into operation. Relay winding 83a is energized to move contact 83b so that the output of receiver 81 is applied to the shifting register 82, sampling pulses being applied to the winding 79a to close contact 79b during the middle portion of the received pulses and shifting pulses being applied to the register 82 to step forward the impulse until the 30 pulses constituting five characters and the check pulses which have been stored in the register 82 are simultaneously transferred to the register 83. An error correcting computer 84 comprises five scanning members 1d, 2d, 4d, 8d, 16d driven by mechanism set into operation by the generator 97 and arranged to examine the pulses corresponding to those examined by the members 1a . . . 16a at the transmitter and the corresponding derived check elements. The impulses detected by the members 1d . . . 16d are fed to odd-even pulse counters 87a ... 87e, and if a check element is in accord with the pulses examined the output of the counters will in all cases be zero corresponding to an even number of pulses. If the examination of the message pulses together with the check element applied an odd number of pulses to a counter, 87a . . . 87e, a positive output is derived from that counter and applied to a corresponding storage unit 88a . . . 88e. If an element of one of the message signals has been mutilated, a positive output is derived from two or more of the units 88a ... 88e, and, for example, by reference to the schematic table, Fig. 3, mutilation of element 7 will result in pulses from units 88a, 88b, 88c, and mutilation of element 30 will produce pulses from units 88b ... 88e. The combination of pulses from the units 88a. . . 88e is applied to a digit decoding network 89 which determines uniquely the element which is faulty and applies a positive pulse to an element modifying circuit 100 to which the received element is also applied from the register 83, a circuit 100 being provided for each of the message elements. One of the circuits 100 is shown in Fig. 6, the pulse from the register 83 and from the network 89 being applied to terminals 101, 102 connected via rectifiers 103, 104 to junction 105 and the grid 114 of a valve 111. Positive battery is connected via resistor 110 to the cathode of the valve 111 and also via rectifiers 108, 109 to the rectifiers 103, 104 respectively. If a space or a pulse is applied to terminal 101 and a pulse is not applied to terminal 102 indicating that mutilation has not occurred the valve 111 is left non-conducting or made conducting respectively when a timing pulse is applied to the second grid 113, and in the latter case, a negative pulse from resistor 112 is inverted at 115 and applied as a pulse to the corresponding member of the register 91. If a pulse is applied to terminal 101 and a pulse also to 102, indicating mutilation of the element, valve 111 is left non-conducting and a pulse is not applied to the register 91 and if a pulse is applied solely to terminal 102, the valve 111 is made conducting so that a pulse is applied to unit 91. An error in an element of the check group operates one only of the units 88a . . . 88e and does not lead to correction of the message elements. After the elements of the message signals, corrected, if necessary, have been stored in the register 91 they are passed with the insertion of start and stop elements, by a distributer 92 which is set into operation by mechanism 93 controlled by the generator 97, to a printing telegraph receiver 95. The generator 97 also produces impulses for resetting the registers 83, 91 and the computer 84. Specification 710,258 also is referred to.