Patent GB-916918-A (GB)
Title: Improvements in or relating to telegraph systems
IPC: H04L1/00
Filed: 1959-02-16
Abstract: 916,918. Frequency shift radio signalling. PHILIPS ELECTRICAL INDUSTRIES Ltd. Feb. 16, 1959 [Feb. 19, 1958], No. 5289/59. Class 40 (5). [Also in Group XL (b)] In a telegraph receiver for the reception of phase or frequency-shift signals a device is provided for producing a control voltage which varies with the level of the incoming signals and which varies the threshold of a testing device in a sense opposite to the level variations. In this manner the probability of interference, e.g. sudden noise, producing an undetected inversion of a signal element is materially reduced. The operation of the transmitter and receiver system is according to known methods and comprises repeating signals stored in delay registers at both transmitter and receiver in order that the characters finally printed are given in their correct order in spite of the break in transmission of the message. Signals arriving at a station ST 2 , Fig. 1, from a station ST 1  pass through filters and demodulators 6, 7 to combiner 17. The signal at the output of the combiner 17 passes via a delay register 9, which has a delay of one character period, i.e. 140 m.sec., to a printer 11. The signal also passes the push-pull rectifiers 18, 19, which form the testing device, and a control voltage generator 21. The output of the latter varies in inverse proportion to the level of the signal fed to it and biases the push-pull rectifiers 18, 19. If the signal level is less than the threshold value of these rectifiers the signal is not passed to the printer and a unit 20 emits a pulse to a timing circuit 13. The timing circuit is fed by a pulse generator 14 which gives a pulse at the start of each code element, i.e. 7 pulses per character, the generator being synchronized with the transmitter. The generator also feeds the trigger unit 15 with a pulse at the end of each character. Immediately a pulse is received from unit 20 switch 8 reverses and the timing circuit starts to count the pulses from unit 14. After 14 pulses, i.e. two character cycles, switch 8 reverts to its normal position. If, when a pulse from unit 14 is received by trigger 15, unit 13 is counting switch 10 is opened. It is thus arranged that this switch cannot be opened if the delay register 9 is transferring a signal to the printer when switch 8 is opened, i.e. switch 10 can be opened only at the end of a character. The delay register 9 thus contains up to 7 code elements and these circulate until switch 8 returns to rest. Unit 20 also causes a transmitter Z 2  to send a signal to station ST 1  where a timing circuit 16 is operated. This reverses switch 2 for 14 element periods and stops the transmission of signals from the signal producer 1. A delay register 4 has a delay of 2 code characters and the elements circulating in this are transmitted and received by station ST 2 . The above timing arrangements ensure that the characters fed to the printer 11 arrive in the correct order in spite of the blocking of some of the signals by the testing device. The control voltage generator 21 comprises a valve 26, Fig. 2, fed with the demodulated signals and with signals from an oscillator 36. The signal frequency output passes via a lowpass filter 29 and via rectifiers to the valve grid where it operates to vary the mutual conductance of the valve to maintain its output constant. The oscillator frequency output will thus vary inversely as the signal input and this output passes via a selective filter 37 to a push-pull rectifier 38 which gives positive and negative bias voltages inversely proportional to the signal level to bias the rectifiers 18, 19.