Patent GB-683843-A (GB)
Title: Improvements relating to code signalling systems
IPC: H04L1/20
Filed: 1950-07-28
Abstract: 683,843. Frequency-shift telegraphy. STANDARD TELEPHONES & CABLES, Ltd. July 28, 1950 [Aug. 8, 1949], No. 18893/50. Class 40 (v). [Also in Group XL (b)] In a code signalling system in which transmission is effected by mark and space signals, an error indicating device provides an alarm if no signal is received or if both signals are received simultaneously. The invention is described as applied to a system utilizing a radio link with frequency-shift signalling. Fig. 1 shows an embodiment employing relays. The transmitting equipment comprises a teleprinter 1, frequency-shift modulator 2 and transmitter 3. The receiver 4 includes filters 5 and 6 which select the mark and space signals and pass them to rectifiers 7 and 8 to produce direct-current signals. The rectifiers are connected in a bridge circuit including resistors 28, 29, and a polarized relay R is connected across the diagonal. An alarm relay A<SP>1</SP> is connected in a circuit to the moving contact of the polarized relay. Upon receipt of either a mark or a space signal the corresponding rectifier delivers an output and the bridge is unbalanced causing relay R to move between its mark and space contacts and operate the single - current receiving teleprinter appropriately. Alarm relay A<SP>1</SP> is held operated so that its contacts remain open and no alarm is given. Upon absence of signal, or if both rectifiers deliver an output, the bridge is balanced and relay R releases, its contact returning to the central position. Relay A<SP>1</SP> then releases and an alarm is given. A modification, Fig. 2, employs electronic circuits. Rectifiers 7 and 8 in Fig. 1 are connected to terminals 32 and 33, terminal 32 becoming positive with respect to terminal 33 during a mark signal and the condition reversing during a space signal. The signals operate a regenerative repeater 37. The signals are also applied to the grids of valves V<SP>1</SP>A, V<SP>1</SP>B between the anodes of which are connected terminals 46 and 47 of a bridge rectifier 48. Terminals 51, 52 connect with the grid and cathode circuit, respectively, of a valve V3 in the anode circuit of which is an alarm relay A. During a mark signal, terminal 32 becomes positive and valve V<SP>1</SP>B conducts whilst V<SP>1</SP>A cuts off. Rectifier X<SP>1</SP> provides a short-circuit and the potential of terminal 52, which is connected to earth through a high resistance 69, becomes equal to that at the anode of valve V<SP>1</SP>A, namely H.T. potential. Rectifiers X<SP>3</SP> and X<SP>4</SP> act as a potentiometer between the anodes of valves V<SP>1</SP>A and V<SP>1</SP>B and terminal 51 acquires a potential less than that of terminal 52. Under these conditions the grid 49 of valve V<SP>3</SP> is negative with respect to the cathode and the valve cuts off, preventing the alarm relay from operating. During a space signal, similar conditions obtain with rectifier X<SP>2</SP> conducting to connect terminal 52 to the anode potential of valve V<SP>1</SP>B. Upon absence of a signal or upon receipt of both signals, the anodes of valves V<SP>1</SP>A and V<SP>1</SP>B become equal in potential and likewise terminals 51 and 52. Valve V<SP>3</SP> thereupon conducts and the alarm relay operates. To prevent operation of the alarm during faults which are of insufficient duration to cause false operation of the receiving instrument, a trigger circuit V<SP>2</SP>A, V<SP>2</SP>B having an adjustable time delay of 1-5 milliseconds set by resistor 66, is triggered from terminal 51 and holds V<SP>3</SP> non-conducting by a negative potential applied to the second control grid 64.