Patent GB-823681-A (GB)
Title: Improvements in or relating to picture transmission systems
Inventor: PARKER ERIC
IPC: G01S7/00
Filed: 1955-10-28
Abstract: 823,681. Television. DECCA RECORD CO. Ltd. Jan. 18, 1957 [Oct. 28, 1955], No. 30965/55. Class 40 (3). [Also in Group XL (c)] In a picture transmission system in which the subject is scanned to produce signals to modulate the beam of a cathoderay tube at a distant receiver, the scanning of the picture image is effected in one co-ordinate direction at a rate synchronized with an alternating power supply frequency, so as to be scanned at that frequency, or at a multiple, or at a submultiple thereof, and in the other co-ordinate direction at a rate derived from and less than said power supply frequency, the period of scan in said other co-ordinate direction being between 1 sec. and 1 minute. In the arrangement described for the transmission of a radar plan position display derived from the elements 10, 11, 12 and 18 and formed on the screen 15 of a C.R.T. 14, the image on the screen 15 is spirally scanned by coils 22, 23 fed with currents in phase quadrature from a phase splitter 20 supplied with A.C. voltage at a fixed frequency e.g. 50 c.p.s. from a mains supply 25, 26. A motor 28 driven from the supply 25, 26 rotates through a reduction gearing, the sum 27 of a potentiometer 24 at, for example, a rate of 10 revolutions per minute, so that the spiral scanning of the image is effected in 6 secs. with a total of 300 spiral convolutions. The arrangement for reproducing the image in the tube 33 is generally similar to the scanning arrangement at the transmitter, apart from the incorporation of a phase-shifter 44 for orientation of the image and contacts 36 operated to synchronize the potentiometer 37 with the potentiometer 24. In a modified arrangement, Fig. 2 (not shown), using a channel of a V.H.F. transmission system, the transmitted signals carry a synchronizing modulation at the frequency of the power supply and if the receiving arrangement does not include a mains supply frequency, the output of a final amplifier is also fed to a filter circuit operating to separate the synchronizing frequency which is utilized to control a generator in lieu of the mains 34, 35 for the scanning operation. A modification of Fig. 1 is shown in Fig. 3, in which a motor 73 driven from the mains 71, 72 in conjunction with gearing 74 rotates a deflecting coil 75 at a rate less than the supply frequency and through a further reduction gear 76 rotates the arm 77 of a variable potentiometer 78 at a rate so that a predetermined number, for example 300, of convolutions are employed in the complete scan of the image. In a further modification of Fig. 1, Fig. 4 (not shown), the amplitude of the mains supply voltage applied to the phasesplitter is varied by an amplitude control circuit which is operated by a cycle counter fed with the A.C. supply and arranged to reset the amplitude control to its initial value after a number of cycles corresponding to the number of rotations or convolutions in a complete scanning cycle. To ensure synchronism, the cycle counter may be reset by a synchronizing signal sent at the end of a complete scan. Specifications 664,858 and 823,683, [Group XL (c)], are referred to.