Patent GB-579126-A (GB)
Title: Improvements in or relating to electric signal transmission systems
IPC: H03K7/00
Filed: 1939-11-10
Abstract: 579,126. Pulse modulation; multiplex signalling ; television. STANDARD TELEPHONES & CABLES, Ltd., and BEATTY, W. A. Nov. 10, 1939, No. 29806. [Classes 39 (i), 40 (iii) and 40 (v)] Pulses which are modulated in time such as the various types of pulses modulated by a sound or other wave as described in Specifications 511,222 and 523,575, or television line and frame pulses of different durations, are employed to control the generation of trains of damped high frequency oscillations the initial amplitudes of which represent the information carried by the pulses. The invention is applied primarily at receivers to reconstitute the original wave or to discriminate between line and frame pulses, but the pulses may be converted into damped trains before transmission. It enables the " depth " of time modulation to be made very small so that a high degree of secrecy is obtained, the pulses being called " fluttering " or having a " fluttering edge." A single pulse train may be used for diplex transmission, one message being transmitted by relatively deep time modulation as in the above-mentioned Specifications and the other by a superimposed flutter or fluttering edge. A multiplex system employing cathode ray tubes as distributors is also described. Line and frame pulses may be " flutter " modulated to convey information regarding brightness level, colour or stereoscopic effects, volume compression and accompanying sound as already proposed for deeper modulation in Specification 527,310. The principle underlying the invention will be understood from a consideration of Figs. 2 to 5. If a simple tuned circuit is excited by a pulse of the form shown in Fig. 2, damped high frequency trains of opposite phases are generated as shown in Fig. 3 and if the durations of the pulses are such that the trains overlap as shown in Figs. 4 and 5 and if the trailing edges 43 of the pulses shown in Fig. 4 are variably spaced in time with reference to the leading edges 42 in accordance with sound or other wave amplitudes, the relation phases of pairs of damped trains vary from pulse to pulse so that the amplitudes of the trailing edge trains 41 vary in accordance with the amplitude of the sound wave. Preferably, the maximum time modulation of the trailing edges 43 is arranged to be a half cycle of the high frequency waves which may have a frequency of 1 m.c. Reproduction of the sound wave is obtained from the damped trains 41. A receiver in which generation of the damped trains is used to reconstitute the sound &c. wave is shown in Fig. 7. Transmitted pulses of the form shown in Fig. 4 with fluttering trailing edges are applied over terminals 46 to shock excite a tuned circuit 31, 32 in the input of a valve 48 normally operating as an anode bend detector. The pulses are also applied across terminals 53, 54 in such a manner that the voltage across resistance 51 paralyses the valve during each pulse so that the valve detects only the damped trains 41 to reconstitute the sound &c. wave. If the pulses are short compared with the intervals between successive pulses, the trains generated by the leading edges are almost at maximum amplitude when the trailing edge trains are superimposed on them, so that increased amplitude differences are obtained and the receiver need not suppress the leading edge trains. Modified receiver, Fig. 19, using regeneration to avoid damping of the leading edge trains thereby increasing the amplitude variations in the trailing edge trains. The pulses are applied over terminals 151, 152 and are of voltage sufficient to set back-coupled valve 140 into sustained oscillation, but in the intervals, between pulses the valve ceases to oscillate and damped trains appear in the tuned grid circuit 148, 149 of amplitudes dependent on the relative phasing of the damped and undamped trains. The damping may be varied by a shunt resistance. The succession of undamped and damped trains from the grid circuit of 140 of the form shown in Fig. 21 is applied to pentode 156 the screen grid of which is controlled by the original pulses whereby its output takes the form shown in Fig. 22 and is applied to leaky grid detector 167 which operates past the cut-off point as regards the undamped trains 190 so that its output takes the form shown in Fig. 23 wherein the undamped trains are represented by positive pulses 175 which are removed by valve 176 the grid of which is also controlled by the original pulses. Transmitting. The valve 140 may be used alone as a transmitter of trains of the form shown in Fig. 21. Systems using " fluttering pulses," each pulse being extremely short and fluttered as a whole instead of fluttered at one edge. The circuit for generating the damped trains, Fig. 14, comprises a high-frequency generator 78, a blocking circuit 79 and a variable damping device 82 such as a shunt valve, connected to coupled tuned circuits in the input of valve 88. The short fluttering pulses are applied to the blocking circuit, damping circuit and as high tension to valve 88 so that-high frequency is fed undamped to an operative valve circuit for the duration of each pulse during which sustained oscillations are built up in circuit 83, 84. On cessation of the pulse, the valve is inoperative, circuit 80, 81 is damped, and a damper train follows in circuit 83, 84, the damped and undamped trains being then separated as described with reference to Fig. 19. Correct phase adjustment between the mean position of the pulses and the H.F. wave is necessary. Short fluttering pulses may be obtained from longer ones by passing them through one or more delay circuits and combining the delayed and undelayed pulses as described in Specification 528,192 to form pulses of stepped form, the tops of which are used as effective pulses. The duration of the derived pulses may be accurately adjusted by vernier delay arrangements as described in Specification 519,747. Diplex system. The diplex system uses two synchronized cathode-ray tubes 97, 194, Fig. 15, with strip targets 96, 193 to produce single short pulses when scanned in the direction of the arrow under the control of a saw-toothed wave and at right angles by the two message waves, each operating as described in Specification 523,575. Target 96 is steeply inclined and produces pulses with considerable variations in time occurrence, but target 193 is only slightly inclined and gives a fluttering pulse. The two sets of pulses correspond to the double pulse system described in the above-mentioned Specification, the fluttering pulse caused to lead or lag the other pulse by suitable adjustment of synchronization. The two sets may be transmitted as such and converted in known manner at the receiver into solid pulses of variable duration (one message) with one fluttering edge (second message) or may be converted before transmission. The first message is then derived in the manner known for solid pulses and the second by arrangements similar to Fig. 14. There is a tendency to cross-modulation in the above arrangement which is avoided by fluttering both sets of pulses, this being effected by superimposing a flutter derived from target 193 on the time base which controls the scanning of target 96. The second message is taken from the edges which have a pure flutter and if this is the leading edge it will be necessary to change the phase of the pulses so that it occurs on the trailing edge so that damped trains characterised as above described may be produced. Multiplex system. This system uses a cathode-ray tube, Fig. 16, having spaced pairs of targets 100, 101, one behind the other, of which the larger series 100 operate as a distributer and the smaller series produce variable duration pulses corresponding to the messages when swept by a rotating beam, the radius of the sweep at any moment being dependent on the amplitude of a particular message wave. The transmitting arrangements, Fig. 17, comprise, .message sources 126-133, blocking circuits 118-125, an A.C. source 116 of pulse frequency, circuits 117 for amplitude modulating the A.C. by the message waves and a phase splitting circuit 138 for supplying the modulated wave in quadrature to the deflection system of the cathode-ray tube. When the beam meets the plates 100, impulses are sent to unblock in turn the circuits 118-125 so that the wave form 116 is amplitude modulated in turn by the sources 126-133, resulting in the production of " flutter " modulated pulses from the plates 101, the duration of a pulse being dependent on the momentary radius of the sweep: The plates 101 may be differently shaped to give a different time modulating law for the various messages. The pulses are separated at the receiver by a synchronized cathode-ray distributer and converted into the original waves by the methods previously described. When different time-modulation laws are used for the messages, different high frequencies are required for the damped trains used for conversion. Discrimination between pulse series having pulses of different duration, e.g. line and frame pulses,of 10 and 40 micro-seconds respectively. The pulses are used to generate damped highfrequency trains and it will be clear from the above description that the initial amplitudes of the damped trains from the line pulses differ from those due to the frame pulses. Separation is effected by amplitude filters. The discrimination may be extended to more than two sets of pulses.