Patent GB-1195329-A (GB)
Title: Formation of Stationary Images Corresponding to Images in a Moving Magnetic Recording Member
IPC: H04N5/78
Filed: 1968-11-21
Abstract: 1,195,329. Television. E. I. DU PONT DE NEMOURS & CO. 21 Nov., 1968 [6 Dec., 1967; 8 Nov., 1968], No. 55397/68. Heading H4F. [Also in Division H1] Apparatus for converting primary image recorded on a moving magnetic tape 10 to secondary images each of which may be converted to an optical image, comprises a receptor surface 11 having magnetic properties including coercivity lower than that of the magnetic tape and greater than the maximum primary image field, the receptor surface being positioned adjacent the magnetic tape so that the local surface magnetic field of each primary image is applied to the receptor surface. The magnetic tape is continuously moved relative to the receptor surface and bursts of magnetic field are intermittently applied to the receptor surface by means of biasing means 22 to transfer the primary image to the receptor surface when the primary images are in the desired position relative to the receptor surface. Receptor surface 11 includes a thin ferromagnetic film 13 e.g. nickel-iron alloy, laid on a rigid transparent substrate 12 and coated with a thin protective surface 14. A pressure pad 15 holds the tape in intimate contact with the receptor surface 11. The biasing means includes a coil 22 wound on a rectangular block of non-magnetic, non-conducting material and is intermittently energized in response to trigger signals impressed at appropriate positions on the magnetic tape and detected by playback head 3. The trigger signals identify the position of each magnetic primary image recorded on the tape, for example when motion pictures are to be reproduced, the sprocket holes on the original may be employed to generate the magnetic trigger signals in the proper positions on the magnetic tape. Other biasing means may be provided, for example current may be passed intermittently through the receptor surface itself or through a conducting surface which is contiguous to the receptor surface. To display each secondary image, plane polarized light is directed upon the receptor surface 11 in an area equal to the area of each primary image and the reflected light is modulated by the Kerr magneto-optic effect, the plane of incidence of the incident collimated plane polarized light beam being arranged to contain the easy axis of magnetization of the receptor surface. The visible image formed by light which has passed through analyser 20 and image forming lens 21 may be viewed directly, photographed or detected by an image orthicon or vidicon tube 19. Synchronism between the tape motion and the field repetition rate of a vidicon tube 19 is not necessary since the vidicon integrates light flux incident thereon and the secondary image in the receptor surface remains stored therein between framing pulses. Preferably the tape transport and the biasing means are activated to provide a framing rate between 24 and 8 frames per second for flicker-free display, the vidicon working at 60 interlaced fields per second. The easy axis of magnetization of the receptor surface is always arranged to be parallel to the magnetization of the tape which may be parallel or transverse to the long direction of the tape. The biasing field bursts may be applied in a direction which is parallel or transverse to the easy axis of magnetization of the receptor surface and each burst may comprise an oscillating magnetic field which has a field strength which is gradually reduced from a maximum level producing magnetic saturation of the receptor surface to a minimum level which is below the coercivity of the receptor surface to produce anhysteretic remanent magnetization which is a function of the local magnetic field of each primary image. Alternatively each burst may comprise a unidirectional field having an initial polarity and an initial amplitude greater than the coercivity of the receptor surface to erase the previously transferred secondary image, the unidirectional field being switched to the opposite polarity with an amplitude less than the coercivity of the receptor surface to produce remanent magnetization of the receptor surface which is a function of the local surface magnetic field of each primary image. U.S.A. Specifications 2,915,594 and 2,793,135 describe methods of recording optical images in magnetic form on the magnetic tape.