Patent GB-774471-A (GB)
Title: Improvements in or relating to secret signalling systems
IPC: G01S17/02
Filed: 1951-01-12
Abstract: 774,471. Supply system for space discharge apparatus. TOUVET, G. Jan. 12, 1951, No. 883/51. Class 38 (4). [Also in Group XL (b)] An optical signalling system incorporates one or more transmitters wherein visible or invisible optical radiation is projected in a given direction from one or more electric discharge tubes excited by oscillatory electric currents which can be modulated so that the radiation carries the intelligence to be transmitted, the overall output of the optical radiation being maintained at a constant level so that the radiation does not exhibit an intelligence-frequency envelope. The total optical output can be maintained constant by keeping the total exciting current constant. The following systems can be used: (1) a double-carrier system with variation in the ratio of the amplitudes of the two carriers for use particularly in telegraphy and telephony; (2) a double-carrier system in which the intelligence modulation in one carrier is 180 degrees out of phase with that in the other particularly for use in telegraphy and telephony; (3) a double carrier system for telegraphy in which the frequency jumps from one value to another ; (4) a system using frequencymodulation of infra-red radiation; (5) a system using a frequency-shift keyer which shifts the radio-frequency between two frequencies; and (6) a system in which the optical radiation is modulated by shifting the spectral band excited by radio-frequencies as referred to in Specification 774,475 particularly for use in telegraphy and telephony. In systems (1)-(5) the discharge tubes can be conventional and need not be exerted solely by radio-frequencies-supersonic or audio-frequencies can be used. In Fig. 1 the discharge tube 1 containing a rare gas such as xenon at a low pressure of, for example, 3-60 mm. is mounted at the focal point of mirror 2 and its radiation enters filter 3 which passes only infra-red radiation. Tube 1 is the anode load for amplifier 4. Intermediate power amplifier 5, buffer 6 and oscillator 7 provide driving power at the desired voltage and frequency. Modulator 8 is used when the R.F. is audio-modulated and modulator 9 when the R.F. is modulated by a second R.F. The modulation system can be applied to either the intermediate power-amplifier 5 or the final power-amplifier 4 by the transfer-switch 12. Frequency-shift keying 10 or frequency modulation 11 can be used. The discharge tube 1 can be of conventional form or as described in Specification 774,475. Starting and matching circuits as described in Specifications 774,474 and 774,477 can be used so that when discharge in the tube stops, turning the line supply off, the device returns automatically to the initial condition, avoiding any manual operation and constituting a protective feature. In Fig. 12 two similar R.F. power stages 4 and 4<SP>1</SP>, as shown in Fig. 1, are fed and turned to different frequencies and modulated by an audio-amplifier 24 so that the total optical output from tubes 1 and 11 is constant. In Fig. 13 two R.F. oscillators 30 and 31 are tuned to different frequencies and the two R.F. outputs are modulated at 32 and 33, 180 degrees out of phase with each other, by a centre-tap transformer 34 connected to the output of an audio-amplifier 35. Modulated stages 32 and 33 supply driving voltage to two power stages 36 and 37, the plates of which are connected to one electrode of a gas discharge tube 38, the other electrode being connected to H.T., the arrangement providing telephony with a constant optical output. Fig. 14 shows a transmitter similar to that shown in Fig. 13, except that the output of the power stages 36 and 37 excites the tube 38 through two tank circuits 60 and 61. In Fig. 15, corresponding to Fig. 9 of Specification 774,475, is shown a rare-gas-filled tube 40 forming part of a R.F. oscillator 41 is conjunction with the R.F. circuits 42. Modulator 43 is used when continuous modulation is desired. Pulse generator 44 is used, when pulse operation is desired, to control the oscillator circuit 42 and tube 40 which produce the corresponding optical pulses. The tube may form part of the inductance of the tank circuit or may generate oscillations through possessing negative resistance. The circuit shown in Fig. 15 is particularly suited for use in system (6) referred to above in which the spectral composition of the optical output is varied as referred to in Specification 774,475. The tube can have a sinusoidal input or can be operated in Class B or Class C or by very short D.C. impulses of the order of 1 microsecond, containing R.F. transients as referred to in Specification 774,475, The variation in the emitted optical wavelengths can be compensated by means of a second similar tube disposed in front of the main tube and excited in such manner that the total optical output is of substantially constant amplitude and spectral composition. The R.F. modulation is chosen so as to shift the band of optical radiation of the first tube in one direction and another modulation frequency for the second tube is chosen so as to shift the band of radiation of the second tube in the opposite direction. The two bands can overlap and a substantial compensation is obtained with respect to variation in spectral composition. Specification 774,472, [Group XL (b)], also is referred to. The Specification refers to applications for providing recognition signals to identify optical transmitters such as I.F.F. retransmitters, for producing a general call frequency, to systems in which special frequencies are assigned to different vessels, to duplex and multiplex communication systems and to remote control of optical systems such as lock-in and automaticfollowing circuits.