Patent GB-723027-A (GB)
Title: Improvements in or relating to telegraph repeaters
Inventor: WRIGHT ESMOND PHILIP GOODWIN
IPC: H04L25/04
Filed: 1953-01-23
Abstract: 723,027. Code telegraphy. STANDARD TELEPHONES & CABLES, Ltd. Jan. 23, 1953 [March 17, 1952], No. 6853/52. Class 40 (3). A telegraph system comprises means for transmitting and receiving combinations of elements of two kinds, each of which combination consists only of a start element and a constant number of permutable intelligence elements, the start element of any signal combination being always of opposite kind to that of the last permutable element of the immediately preceding combination. Thus, considering the conventional seven-unit startstop teleprinter code, the five permutable elements are transmitted without change and are then followed by an interval of at least another element's duration (normally occupied by the stop signal) during which the line polarity remains the same as the last permutable element. Thereafter a pseudo-start element for the next combination is sent of opposite polarity to the last permutable element. According to one feature, the convention according to the invention permits a saving in transmission time since the seventh element now conveys no information and may be eliminated reducing the time for a code combination from 140 to 120 ms. It is stated that the reduction may be effected by recording the combinations on a medium, such as a magnetic tape and then reproducing them so that the seventh element is eliminated. A complementary recording and reproducing equipment is necessary at the receiving station so as to spread the signals out again to 140 ms. before converting them back to the ordinary teleprinter code. According to an alternative feature the seventh element may be used as a sixth permutable element conveying further information, such as a case shift signal individual to each combination, or as a signal for an error-checking system. The method of error-checking envisaged comprises the steps of counting the number of space elements in each combination (not including the start element) and inserting a space for the seventh element if the number is odd and a mark if the number is even. At the receiver a similar evaluation is made and the two check elements are compared to see if an error has taken place. Figs. 3 and 4 illustrate the transmitter and receiver respectively of an embodiment arranged to employ the error-checking system described above. The invention is applied to a coldcathode regeneration repeater system of the type described in Specification 692,411 in which operation at each station is controlled by a 5 kc. pulse source (P+, P -) triggering a time scale circuit comprised, as shown in Fig. 3, of multi-gap discharge tubes C1, C2, C3 arranged respectively to count every one, every tenth and every hundredth pulse. Potentials for opening gates at selected intervals after the start of a count are derived by taking connections from appropriate combinations of gaps in the three tubes. Gates are indicated throughout the diagrams by the prefix G, and the times in milliseconds at which potentials are applied to them from the time scale circuit indicated thus: T2, T30, T138, &c. The gates may comprise the type disclosed in Specification 636,700, [Group XL (c)]. Considering first the transmitter, Fig. 3, the normal seven-unit startstop teleprinter code combinations are applied to the mark and space input leads, and the converted combinations are generated by a twocondition output device F3. Gates G1 and G2 examine the input signal and pass 5 kc, pulses in dependence on the signal to lead M or S. The start signal initiates the cycle of operation of the time scale circuit by triggering a twocondition device F1 and after 10 ms. gates G7 and G8 examine the state of device F3 and reverse its condition to generate the pseudostart element of opposite polarity to that last generated (i.e. the last element of the previous combination). After 30, 50, 70, 90 and 110 ms., i.e. at the mid-points of the five incoming permutable elements, gates G11 and G12 examine the conditions on the M and S leads and pass pulses to set output device F3 in like manner. The incoming signal is also examined at 25, 45, 65, 85 and 105 ms. to determine the number of space signals by gate G3 connected solely to the S lead. The gate controls a twostage counting device C4 which is reset to register an "even" total at 2 ms. at the beginning of each cycle is thereafter triggered back and forth by each space signal. If the total is an even number gate G10 is conditioned and at 130 ms. (i.e. the seventh element) opens to set the output device F3 to mark, if not already at mark, whereas if the total is an odd number gate G9 sets the output device to space. At 138 ms. a potential is applied from the time scale circuit to reset device F1 and the operation then stops awaiting the receipt of the next start signal. At the receiver, Fig. 4, operation proceeds in a like manner, the condition on leads M and S being examined by G19 and G20 during the five permutable elements to control the condition of the output device F5. In order, however, to restore the normal teleprinter combination, device F5 is forcibly set to space at 10 ms. to reinsert the invariable start element and is likewise forcibly set to mark at 130 ms. to reinsert the stop signal. The presence of an error is detected by a gate G13 and counter C5 which operate in manner similar to the corresponding elements at the transmitter to condition a pair of gates G23 and G24. At 130 ms. these gates examine the condition on the M and S leads and any discrepancy between the condition of counter C5 and the seventh element causes a two-condition device F2 controlling a visual or aural alarm to be triggered. In order that operation may be initiated correctly by the pseudo-start signal the connection to the two-condition device F4 controlling the time scale circuit extends through gates G21, G22, conditioned by counter C5. If at the end of a combination the counter stands at " even " indicating that the seventh element is a mark and therefore that following pseudo-start element will be a space, gate G22 is conditioned, whereas gate G21 is conditioned when the counter stands at " odd " indicating that the pseudo-start element will be a mark. Short start suppression. In order to prevent the receiver from operating to interference, a modification, Fig. 5 (not shown), is arranged to examine the line condition 9ms. after the receipt of a supposed pseudo-start element and, if the line condition has changed, to stop the operation of the time scale circuit. Figs. 1 and 2 (not shown), illustrate a system generally similar to that shown in Figs. 3 and 4, but without the facility for error checking. Specification 710,258 also is referred to.