Patent GB-635346-A (GB)
Title: Improvements relating to ultra-rapid radiotelegraphy systems
IPC: H04L27/26
Filed: 1946-08-27
Abstract: 635,346. Printing telegraphy. SOC. FRANCAISE RADIO-ELECTRIQUE. Aug. 27, 1946, No. 25659. Convention date, May 22, 1942. [Class 40 (iii)] A multi-channel telegraph receiving system operating to a code in which each character is represented by a signal of two different frequencies comprises means for transforming the two frequencies of each signal into two corresponding voltages, a cathode-ray tube the beam of which is deflected by these voltages to a unique position for each received signal and distributing means responsive to the deflections of the beam and acting successively upon printers to record the characters corresponding to the beam's positions. Transmission. Each character is represented by two frequencies transmitted simultaneously and selected. one from each of two groups of frequencies as described in Specification 316,298, the two groups being separated by a gap in the frequency spectrum. These frequencies modulate a carrier in amplitude, but it is stated that frequency modulation or un-modulated carrier may be employed. Reception. Amplitude-modulated signals are received and detected by diversity receivers 2, Fig. 1, the strongest signal being fed to resistor 4. A.V.C. is supplied by leads 5 and the output from resistor 4 is amplified and filtered at 6 and separated by the top and bottom filters 7 into the original two frequency groups. The signals are then amplitude-limited and applied to modified Seeley discriminator circuits 9 whose D.C. outputs are representative of the frequency of the received signal. These D.C. voltages are used to deflect the beam of a cathode-ray tube 12 to a co-ordinate position on the screen characteristic of the character transmitted. Each discriminator circuit, Fig. 2, has a relay R which is operated when a signal frequency is received and the circuit is unbalanced thereby. Contacts a and b respectively are opened when relays R of each of the two discriminator circuits 9 operate. Unless therefore one signal frequency in each group is effectively received, either contact a or b short-circuits the output U and no deflection of the cathode-ray beam takes place, thus providing an error indication. A third filter 7<1>, Fig. 1, is tuned to the gap between the two signal-frequency groups and the relay R3 is therefore energized by interference, but not by a pure signal, closing contacts c, Fig. 2, and again providing an error indication. Conversion to five-unit code. The characteristically positioned C.R.T. spot is optically projected on to the mosaic 16 of an iconoscope 13, Fig. 4. A receive distributer 23 is divided into four channel segments 1 to 4 and a control segment 5, and rotates once per letter (25 ms.) in synchronizm with the transmitter distributer. Each channel segment is divided into five rings, each of which is divided into 32 circumferential elements. Five brushes 27 co-operate with contacts placed in each ring, and in each position of the distributer the brushes receive an earth from one to five contacts arranged to represent the spaces in a particular five-unit code combination. These earths are applied to the control grids of five gas-filled tubes 28. A contact 35 on the control segment 5 co-operates with two fixed brushes to brighten the beam of the C.R.T. 12 during the centre 10 ms. only of a 20 ms. signal, during which period a corresponding square of the mosaic is charged. At the end of this 10 ms. a control contact 24 co-operates with a fixed brush to start the sweep generator 25 of the iconoscope 13 which completes its scan in the next 15 ms., producing, at a time dependent on the position of the C.R.T. spot, a pulse in the collector lead 19 which is applied via an amplifier 20 and channel distributer 33 to a channel resistor 30 and to all control grids of the five gas-filled valves 28. At this instant the distributer 23 has rotated to a point where those grids are earthed which correspond to spaces in the five-unit code combination of the character being received, so that only the remaining mark tubes 28 conduct due to the pulse from resistor 30 and operate associated magnets to print in a known manner the desired character in a five-unit code printer I. The iconoscope impulses due to following characters received are fed in turn to other printers by lead 19 and the channel distributer 33. When a character has been printed a printer contact 31 applies negative battery to all grids of the gas-filled tubes 28 which revert to their normal non-conducting state. Since the iconoscope 13 is charged only during the centre 10 ms. of a signal lasting 20 ms., an error in synchronism of 5 ms. between the transmitter and receiver distributers still permits correct functioning of the system. By using a crystal oscillator to control each of these distributers it will be many hours before the system requires re-synchronizing, which may be done by repeatedly transmitting a predetermined character and adjusting the control brushes of the receiver distributer 23 for correct reception. By making the time constant of resistor 21 and condenser 22 sufficiently long, stray charges on the iconoscope, due e.g. to interference, will not charge condenser 22 sufficiently to operate the tubes 28. In a modification (Figs. 6 and 7, not shown), the distributer 23, Fig. 4, comprises a translucent drum, with one set of five-unit code combinations marked thereon in opaque squares, revolving around a fixed drum with a vertical slot through which a light beam shines on to an external vertical row of five photo-electric cells. The output of the photo-electric cells, which represents the character illuminated, is applied to the grids of the five gas-filled tubes 28, the anode voltage of which is supplied from the output of the iconoscope impulse amplifier 20. Those tubes 28 which then correspond to marks conduct and print the required character as above. Multiplexing is provided for by connecting the photo-electric cell outputs to the tubes 28 via a five-bank channel distributer. In a further modification, Fig. 8, (not shown), magnets operated by the gas-filled tubes 28 effect the transmission of voice-frequency line-currents instead of printing the character. Echoes. Close-following echoes due to signals arriving by slightly differing paths may lengthen a signal but this has no injurious effect since only the centre 10 ms. of each signal is used. The effect of true echoes arriving after a transit around the earth of 24,000 km. may be nullified by spacing the transmitted signals 50 ms. apart to ensure the arrival of the echo when the C.R.T. beam is suppressed. The dimensions of the receive distributer are correspondingly altered.