Patent GB-652363-A (GB)
Title: Improvements in and relating to multi-channel, privacy signalling systems
IPC: H04J3/04
Filed: 1947-03-24
Abstract: 652,363. Multiplex radio signalling. GENERAL ELECTRIC CO., Ltd., and LEVY, M. M. March 24, 1947, No. 7998. [Class 40 (v)] A multi-channel signalling system, in which privacy is obtained by changing the channels by which individual signals are transmitted, comprises two parallel paths, each arranged to handle all the channels and each containing allocation-changing switches, the two paths operating alternately, and the switching means in the inoperative path only being actuated. Alternatively, a purality of multi-channel paths each providing a different allocation of signals to channels are operative singly and in a recurrent fixed or varying succession. In one embodiment, forming an eight-channel system, the signal source S, Fig. 1, feeds two telephonetype switching devices SW<SP>1</SP>, SW<SP>2</SP>, for modifying the channel allocation. Their outputs are applied respectively to the deflecting electrodes E of two cathode-ray type distributor-modulators M<SP>1</SP>, M2, of the type described in Specification 619,785, which produce interlaced channel trains of time-modulated pulses. A motor-driven master switch MS generates pulses which control the settings of switches SW<SP>1</SP>, SW<SP>2</SP> and controls the generation of synchronizing pulses. These pulses are produced by slots in the modulating plate P, Fig. 2, of the tubes M<SP>1</SP>, M<SP>2</SP>, the slots SS<SP>1</SP>, SS<SP>2</SP> being the main synchronizing slots. At progressively smaller radii a number of different subsidiary slots or combinations of slots can be selected by different voltages applied to the beam deflecting electrode El. A main synchronizing pulse is sent once every five revolutions of the beam and a subsidiary synchronizing signal once every other revolution, the particular combination of signals indicating the coding being employed at the switching devices SW<SP>1</SP>, SW<SP>2</SP>. The master switch MS controls the sequences of voltage applied to electrodes E<SP>1</SP> in tubes M<SP>1</SP>, M<SP>2</SP> by setting switches BS<SP>1</SP>, BS<SP>2</SP> which determine the voltages applied to electrodes a, b, c, d of distributer devices D<SP>1</SP>, D<SP>2</SP>, respectively, which operate in synchronism with the beam rotation of tubes M<SP>1</SP>, M<SP>2</SP>. The main synchronizing signal is selected by the master switch MS through connections H<SP>1</SP>, H<SP>2</SP>. A flip-flop multivibrator F renders the tubes M<SP>1</SP>, M<SP>2</SP> operative alternately, being actuated by synchronizing signals from slots SS<SP>1</SP>, SS<SP>2</SP>, respectively, over leads G<SP>1</SP>, G<SP>2</SP>, and selected by devices FS<SP>1</SP>, FS<SP>2</SP>. Locking circuits L', L<SP>2</SP>, responsive to the current through modulators M<SP>1</SP>, M2, respectively, prevent the operation of devices SW<SP>1</SP>, BS<SP>1</SP>, D<SP>1</SP> and SW<SP>1</SP>, BS<SP>2</SP>, D2, respectively, or interrupt the channels by which the master switch MS effects operation of these devices, unless the current in the locking devices is zero. An alarm signal may be operated by one or both of the devices L<SP>1</SP>, L2 if one setting is maintained for more than a predetermined time. The modulator outputs are fed to radio transmitter, RT. The modulators M<SP>1</SP>, M<SP>2</SP> may be arranged to operate as in Specification 619,785, each electrode E serving for a plurality of channels. Further thermionic distributers, Fig. 3 (not shown), are provided for this purpose. Receiver.-The radio receiver RR, Fig. 4, feeds signals to a selecting device SD which has three output terminals T<SP>1</SP>, T<SP>2</SP>, T<SP>3</SP> from which respectively are obtained the main and subsidiary synchronizing signals and the intelligence signals. The latter are converted from time to duration modulation in circuit PW, which may be of the kind described in Specification 652,361, and then applied to the contro' electrodes of both distributer tubes RD<SP>1</SP>, RD<SP>2</SP>. The beams of these tubes are synchronized to rotate with those of the transmitter modulators. The eight anodes of tubes RD<SP>1</SP>, RD<SP>2</SP> are connected to switching devices SW3, SW4 and thence to low-pass filters LP and the signal utilization circuit SU. The main synchronizing signals control a flip-flop circuit F<SP>1</SP> to render RD<SP>1</SP> operative when M<SP>1</SP> at the transmitter is operative and RD<SP>2</SP> when M<SP>2</SP> is operative. The subsidiary synchronizing signals are applied to a control device CD which causes switches SW<SP>3</SP>, SW<SP>4</SP> to take up positions corresponding to those of SW<SP>1</SP>, SW<SP>2</SP>, respectively, at the transmitter. Switching takes place only in the one of two parallel paths which is inoperative at any time, circuit CD being controlled for this purpose from circuit F<SP>1</SP> over link N<SP>1</SP> A two-way transmission system may be provided so that the change from synchronizing signal SS<SP>1</SP> to SS<SP>2</SP> may be prevented until a signal is received from the distant receiver signifying that F<SP>1</SP>, SW<SP>3</SP> and SW<SP>4</SP> have correctly operated. For further privacy the signals may be inverted and/or wobbled in frequency before being applied to modulate the pulses. The devices D<SP>1</SP>, D<SP>2</SP> at the transmitter may be constituted by a number of valves controlled by rectangular pulses as described in Specification 652,356. The synchronizing signals may comprise a number of oscillations of different frequencies transmitted as time modulation of pulses. The invention may be applied to systems in which each channel is allotted a different part of a frequency spectrum, one channel being allotted to the transmission of switching information, or an unmodulated sub-carrier may be modulated therewith.