Patent GB-983439-A (GB)
Title: Improvements in or relating to rhythmic telegraph systems for the simultaneous transmission of messages in opposite directions
IPC: H04L1/18
Filed: 1963-01-23
Abstract: 983,439. Telegraphy. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Jan. 23, 1963 [Jan. 26, 1962], No. 2879/63. Heading H4P. The invention provides a synchronous telegraph system for the simultaneous transmission of messages in opposite directions in which means is provided for the detection of faulty reception, for the correction of error by repetition, and for the detection and restoration of synchronism, once lost. Error detection and correction.-The system uses a 4-character repetition cycle, characters from unit 1 in transmitter Z1 being sent by frequency shift radio signals to receiver O 2  in one direction, and from Z2 to O1 in the reverse direction. Characters are sent in 5-unit code preceded by a mark bit and followed by a parity bit. At the receiver the signals are separated into mark and space channels 6, 7, and each bit is checked at 19. The characters are checked for parity at 11 and passed to a 7- bit delay where the first and seventh bits are removed prior to actuation of printer 10. If a mutilated bit or a parity error is detected, a repetition request signal is sent to transmitter Z1 and unit 13 opens contact 9, so that printing ceases, for a period of four characters which is determined by the output of a clock pulse generator 14 and counter 15. Upon detection of the repetition request signal Z1 transmits a special signal followed by the three characters last transmitted. In the special signal the middle five bits are deliberately sent mutilated; this mutilation may take the form of a third frequency which lies between the mark and space frequencies or of the transmission of mark frequency for half of each bit and space frequency for the other half. The system is such that if no error is found in the repeated character previously received mutilated it is passed to the printer. If error is again present further repetition takes place. Correction of synchronism.-During repetition the output of bit-checker 19 is fed to a synchronizing unit 20 which, as long as loss of synchronizing is present, initiates further repetition and also pulses counter 15 at the end of each cycle to change its phase until synchronizing is established. The operation and circuitry of units 15 and 20 are given in detail with reference to Fig. 2 (not shown). Unit 20 includes a cascade arrangement of triggers which provides an output when the special character is received. Unit 15 is a seven-stage counter which should be running in synchronism with the received signals. If it is not, and unit 20 gives an output, and further a mutilation is detected in one of the three repeated characters an extra pulse is passed to counter 15, thereby shifting it in phase relative to the special signal. Further repetition is now requested. This action continues until the special signal and the counter are in phase. However, it may be that although the bit phase is now correct, the character phase is not-this would result in the wrong repeated signal being passed to the printer. To permit rectification of such a situation one character in each cycle is marked, e.g. by inversion. The counter 15 must then be phase-shifted until this marked character occurs at the correct timing.