Patent GB-833648-A (GB)
Title: Improvements in or relating to code telegraphy
Inventor: FIFIELD JOHN ARTHUR RICHARD
IPC: H04L1/18
Filed: 1956-04-10
Abstract: 833,648. Code telegraphy. POSTMASTER GENERAL. May 31, 1957 [April 10, 1956], No. 10847/56. Class 40 (3). In a telegraph system employing an errordetecting code constituted by combinations having a constant ratio of marks to spaces in each character signal and having means operative on the detection of an error in a code combination to control the automatic retransmission of the erroneous signal, the receiving apparatus is provided with means for storing a received erroneous signal until the retransmitted signal has been received and for examining the repeated signal combination as it is received in conjunction with the stored erroneous signal combination, and means for providing the corrected version of the transmitted character or detecting an error in the combined erroneous and repeated signal combinations. In the generally known receiving arrangement, indicated in Fig. 1, the signal elements received over a line I are registered on storage devices R1 ... Rn under control of timing pulses from a distributer ED, and on the reception of marking element pulses are passed to a testing circuit T, which in response to a correctly received signal passes a pulse over conductor TA to a translator allowing the combination registered on R1 ... Rn, with the addition of start and stop elements, to be passed to a receiving translating device. If the received signal is faulty, an impulse over conductor TE arrests normal transmission from the local transmitter which is operated to send a signal requesting the retransmission of the incorrectly received signal combination. For a multiplex system the receivers are rendered operative in turn by a channel distributer CD. In the operative receiving circuit arrangement, Fig. 3, the elements of a 7-unit equal-ratio code are registered by a distributer ED on tubes R1 ... R7, each tube being associated, as shown for R1, with gates GA1, GB1 and a storage tube S1. The testing arrangement T<1>, in response to a correct signal passes a pulse over TA1 to the translating equipment, but if an error is detected in a received signal a pulse passes over conductor TE1 to initiate the retransmission of the faulty signal and a pulse through gate G to operate a timing device TC producing a pulse over T1 to transfer the pulses registered on R1 ... to the tubes S1 ... The testing arrangement T<1> also operates to pass a pulse over a conductor TF if the number of marking elements is less than the prescribed number, i.e. 3, in a 7-unit equal ratio code having 3 marks and 4 spaces. If the number of marks is more than 3, a pulse is passed over conductor T M. The pulse from TF through a gate GF restores a circuit MF to remove an inhibiting pulse from the gates GA1 ... and to cut off pulses from the gates GB1 ... A pulse T3 provided by the timing circuit TC during the repetition period in combination with pulses from the distributer ED operates a gate GA1 ..., if a mark is stored on the corresponding tube S1 ... irrespective of the element received in the retransmission. The combination of the stored erroneous signal with the repeated signal normally provides the correct version of the original signal. If the compounded signal is still faulty, the testing arrangement operates to elicit a further retransmission from the distant station. If, however, the number of marking elements is greater than the prescribed number, a pulse from conductor TM through the gate GM operates the circuit MF to apply an inhibiting pulse to the gates GA1 ... and a pulse to the gates GB1 ... so that if a space signal is stored on a tube S1 ... and in consequence no inhibiting pulse passed to GB 1 ... the corresponding gate GB1 ... opens in response to the two further pulses from T3 and the distributer ED respectively to provide an inhibiting pulse on the associated register R1 ... to prevent its operation even if a mark element is received as the corresponding element of the retransmitted character. Similarly, in this case, the correct version of the signal will normally be reproduced, but further transmission will be initiated if the compounded signal is incorrect.