Patent GB-587405-A (GB)
Title: Improvements in or relating to electric signal transmission systems
IPC: H04N1/23
Filed: 1944-06-27
Abstract: 587,405. Copying-telegraphy; television ; thermionic distributers. STANDARD TELEPHONES & CABLES, Ltd. June 27, 1944, No. 12240. Convention date, June 5, 1943. [Classes 40 (iii) and 40 (v)] [Also in Group XXXVTII] A transmitter for electrical signals in coded form comprises a plurality of switching devices each having a start control circuit, a stop control circuit and an output circuit with signal transmitting means connected in common to all the output circuits, means for generating a train of short negative pulses, and means for selectively applying the pulses to the said start and stop control circuits of the switching devices in such manner as to cause the transmitting means to transmit signals in accordance with the code. The system described relates to. the transmission of letters and figures with their reproduction on the screen of a cathode-ray tube or as a permanent record on a tape. The period of transmission of a character is subdivided into a number of time intervals during each of which a number of impulses of alternating current, generally of different frequencies and determined in accordance with the character are transmitted simultaneously or sequentially. The frequency transmission may be replaced by selective pulses sent in succession during the time intervals. As shown in Fig. I negative potential impulses derived from a circuit 12 are passed over a rotary arm 34 and contacts 0 ... 7 to switch numbers attached to the keys of a transmitter 10 through which the switching devices 20a ... 20g are selectively operative during the intervals of time allotted to a character. The operated device operates its corresponding gate valve 22a ... 22g so that the corresponding oscillator 24a ... 24g modulates a carrier wave supplied by source 28 with one of the frequencies f <1> ... f<7>. The circuit 12 includes a valve 30 operated during each revolution of the distributer 14 by a switch 18, and a timing circuit including the condenser 37 ensures the production of negative pulses throughout each revolution of the arm 34. The keys 38 of the transmitter 10 corresponding for example, to the characters E, 2, N are formed with switch numbers 40a ... 40g, 42a ... 42g, 44a ... 44g, Fig. 2, which are interconnected with conductors leading to the contacts 0 ... 7 of the distributer 14 and further conductors to " ON " and " OFF " terminals of the switching circuits 20a ... 20g. The depression of the key 38 corresponding to the letter E, for example, connects negative potential to the " ON terminal of circuit 20a at the " 0 " position and negative-potential to the " OFF " terminal of that circuit at the " 7 " position so that the frequency f is transmitted during the seven intervals of the period for transmission of the character. The switch 40b connects, negative potential to the " ON " terminal of circuit 20b at the " 0 " position and negative potential to the " OFF " terminal of that circuit at the " 1 " position so that the frequency f<2> is transmitted solely during the first interval. The intervals of transmission of the remaining frequencies f<3> ... f<7> are similarly determined by the switch members 40c ... 40g. The switching circuit 20a, which is identical with the remaining circuits, comprises two valves 48, 50 arranged so that when one is operative the other is inoperative. Normally the valve 48 is operative and the application of negative potential to the suppressor grid of this valve makes it inoperative and at the same time makes the valve 50 operative which permits the gate valve 52 to pass oscillations of frequency f<1> to the modulator 26. The application of negative potential to the " OFF " terminal of the circuit renders the valve 50 inoperative thus cutting off the transmission of frequency from the oscillator and at the same time restoring valve 48 to its operative condition. ' At the receiver, Fig. 3, the received signals are applied through a broad band receiver 268 having a tuning circuit 270 corresponding to the range of frequencies f<1> ... f<7> to a valve 274 whose output brings the valve 278 into operation and applies impulses to the control grid 282 of the cathode ray tube 284.' The scanning beam is arranged to be moved relatively slowly, for example, 30 cycles per second, in the horizontal direction and very rapidly in the vertical direction, for example, 3,000 cycles per second,' and a reactance circuit 272 is coupled to the tuning circuit 270 so that the frequency band f<1> ... f<7> is scanned at a rate corresponding to the vertical deflection speed and the occurrence of a particular frequency or frequencies in the received' signal is reproduced by short pulses in the corresponding position(s) of the vertical travel of the beam. In a modification. Fig. 5 (not shown), the received signal is applied through a broad-band receiver to circuits selectively tuned to the frequencies f<1> to f<7> and controlling through amplifiers and detectors a number of relays operating marking means to provide a record on a tape. A modified-transmitter arrangement in which the frequencies or impulses during the successive time intervals are sent sequentially is described in connection with Figs. 6, 7 (not shown) in which the switching circuits are conditioned as in the previous arrangement by the switches operated by the keys but are sequentially operative under control of a valve switching or counting arrangement which is brought into operation by a pulse generated during each revolution of the mechanical distributer shown at 14 in Fig. 1. In the case in which the frequencies or pulses are sent in succession, seven revolutions of the distributer arm are necessary for the transmission of a complete character, and a single frequency could be employed for the complete transmission. The corresponding receiving arrangement is described in connection with Fig. 8 (not shown). The mechanical distributer 14 of Fig. 1 may be replaced by -a valve distributer, Fig. 9, comprising eight valves 326a ... 326h, a starter valve 324 and an auxiliary valve 334. The potential applied to the valve 324 and over the switch 332 when the keys are in their upper position is such that the valve 324 is not operated by the operation of valve 326h. and is not sufficient to operate the valve 326a. The operation of any key opens the switch 332 and closes a switch 330 in the circuit of the valve 334 which operates and transmits a positive pulse to the valve 324 which in turn operates the valve 326a and thereafter the valves 3266 ... 326h in succession, the intervals between the successive operation of the valves 326a ... 326h being determined by the condensers which shunt the lower halves of the resistances in the control grid circuits. The anodes of the valves 326a ... 326h are connected through condensers to the terminals 0 ... 7 associated with the keyboard.