Patent GB-823682-A (GB)
Title: Improvements in or relating to picture transmission systems
Inventor: THOMAS DAVID LANE
IPC: G01S7/00
Filed: 1956-03-07
Abstract: 823,682. Television. DECCA RECORD CO. Ltd. June 7, 1957 [March 7, 1956], No. 35192/55. Class 40 (3). [Also in Group XL (c). In apparatus for the transmission of pictures having a constant or substantially constant background with additional localized information liable to more rapid change, means are provided for repetitively scanning and transmitting the background information at a relatively low repetition frequency together with means for transmitting the localized information liable to more rapid change at a much faster repetition frequency during intervals in the scanning of the background information. The invention is described as applied to the remote transmission of a radar display in which the position of a target is changing rapidly against a background that remains substantially constant. At the radar station received echoes from radar equipment 10-16 are reproduced on a P.P.I. display 14 and are also recorded in a doubleended cathode-ray storage tube 20 scanned synchronously with the display 14. The recording is then read off for remote transmission in a spiral scanning pattern under the control of a low-frequency sinusoidal signal source 24, e.g. the mains supply, which is applied to deflecting coils 23 for the reading beam through a sawtooth modulator 26 having a recurrence frequency very much less than that of source 24 and phase splitter 25. The video output from tube 20 is applied to a gate 28 (see below). A target marker generator 19 operated by a target marker control 18, which may incorporate auto-following or rate-aiding control systems, is employed to mark a selected target on display tube 14 and to generate a signal indicative of the range and bearing of the target. The signal conveniently takes the form of sine wave at the frequency of source 24 which is derived through a unit 35 and modulated in a stage 36 so that its amplitude represents range and its phase represents bearing. The output from modulator 36 is applied to a gate 37. In accordance with the invention, gates 28 and 37 are operated alternately by means of a gate control unit 42 synchronized from source 24 so as in alternate cycles of the supply to cause the transmission of signals from storage tube 20 and signals from modulator 36 representing the selected target. The substantially constant background is thus transmitted at a low repetition frequency determined by the saw-tooth modulation component of the spiral scan whilst the target information is transmitted at the faster repetition rate determined by source 24. The transmission comprises successive spiral convolutions of background information alternating with the selected target information. Since the backgrotmd information is transmitted at a low repetition rate and the target information relates to only a single target both signals may be transmitted over narrow band, e.g. audio, channels. As illustrated, the signals are transmitted by V.H.F. transmitter 30, the output from gate 28 being impressed directly on the transmitter and the output of gate 37 being modulated on a sub-carrier from source 39. To permit the synchronous reproduction of the display at the remote position, the saw-tooth signal from modulator 26 and a reference sine beam signal from source 35 are also added to the transmission as modulations of subcarriers from source 33 and 41 respectively. At a remote receiver, Fig. 2, the transmitted signals are separated by filters and detectors 52-55. The background signals are applied through amplifier 71 to modulate a display tube 63 whilst the saw-tooth voltage is applied through a shaper circuit 73 to modulate the amplitude of the reference sine wave signal which is applied through limiter 56 and filter 57 to modulator 73. The modulated sine wave thus produced is then applied through gate 74 and phase splitter 61 to control the spiral scanning of tube 63. The target information signal firstly is amplitude detected in stage 58 to derive the range information which modulates the reference sine wave in stage 59 and secondly is amplitude limited at 65 and compared in phase with the reference sine wave in comparitor 67 to control the production of a pulse by generator 68. The sine wave from stage 59 is applied through gate 60 to phase splitter 61 and causes the production of circular scan at the target range whilst the pulse from generator 68 is applied to modulate the beam at the target bearing. An indication thus appears at the target position. In accordance with the invention the background and target information appears alternately and gates 74 and 60 are controlled accordingly by a gate control 70 synchronized from the reference sine wave through filter 57. In a second embodiment, Figs. 3 and 4 (not shown), a simplified communication link employing only two channels is employed, the background and target information passing alternately over one channel and the corresponding display scanning information passing alternately over the other channel. Modifications are referred to in which the target information is transmitted less rapidly, e.g. after a plurality of cycles of background information. The invention is particularly suitable for the transmission of the display from a fixed radar installation to approaching ships or aircraft. A first communication link is employed for the background information which, in the case of aircraft, is preferably transmitted on two or three different range scale whilst a number of further links are employed to transmit to each ship or aircraft using the system the more rapidly changing information about its own position. Specification 823,681 is referred to.