Patent GB-389117-A (GB)
Title: Improved method and apparatus for recording and reproducing electrical signals by means of a magnetic element
IPC: G11B15/50
Filed: 1931-09-10
Abstract: 389,117. Electromagnetic soundrecording apparatus. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.-(Assignees of Hickman, C. N. ; 3536, 79th Street, Jackson Heights, and Wente, E. C. ; 245, West 104th Street, both in New York, U.S.A.) Sept. 10, 1931, No. 25374. Convention date, April 15. [Class 40 (ii).] A telegraphone for recording or reproducing electrical signals on a magnetic tape recordcarrier includes a polarizing magnet which, during recording, magnetizes the tape transversely, and a translating magnet the flux from which includes both a direct component, to de-magnetize the tape until the residual magnetism is substantially zero, and an alternating component corresponding to the signals to be recorded. As shown by Fig. 2, a tape 11 of high magnetic retentivity is moved slowly between a pair of polarizing magnets 26 and a pair of recording or reproducing magnets 27. The magnet-supporting structure, Fig. 3, comprises a non-magnetic member 69 which has openings to receive U-shaped non-magnetic pole-piece guides 71, 72. A slot 73 to allow passage of the tape 11 is formed in the member 69 to a depth such that the arms of the guides 71, 72 are also cut. The magnet pole-pieces comprise a U-shaped non-magnetic casing 74, Fig. 15, which receives the magnetic laminations 75, 76. The assembly is cut along the line 79 to form companion pole-pieces. The magnetic pole-pieces are of a material having a substantially zero magnetic retentivity and, in addition, during reproduction, a small demagnetizing current is passed through the coils 35, Fig. 2, in order to reduce the noise level. The translating magnet pole-pieces may be made of molybdenum permalloy and the polarizing pole-pieces of cobalt or of molybdenum permalloy with pole tips of cobalt iron. The polarizing pole-pieces may be from 6 to 10 mils. in thickness and those of the translating magnets 27 about 2 mils. thickness, for a tape speed of 8 to 12 inches per second. The efficiency of the translating magnets is increased by providing a yoke 80, Fig. 3, the laminations 81 of which are cemented in place in a member 84, to provide a closed magnetic circuit for the alternating flux corresponding to the signals. The yoke is removably held in place, with the ends of the laminations abutting the surfaces 85 of the pole-pieces, by members 82, 83 fitting tightly over the guide 71. A wire spring 86 is sprung up over the ends of the pole-pieces to hold them in contact with the tape, or alternatively a spring 58 may be used for this purpose. Grooved guiding rollers 67, 68, Fig. 6, may be provided on either side of the magnets to prevent lateral movement of the tape 11. The roller 67 is fixed, while the roller 68 is pivoted at 89 and held in contact with the tape by a spring 90. Damping pads 28 are pressed against either side of the tape by a spring support 93. Sounds to be recorded are picked up by a microphone 30, Fig. 2, and electrical current variations are transmitted to an amplifier 32, the switch 31 being in the upper or recording position. From the amplifier, the currents are taken through a transformer 33 to the coils 34 of the magnets 27 which also have depolarizing coils 35 fed by current from a battery 36. Coils 37 form the windings of the polarizing magnets 26. As the tape 11 is moved, it is brought to a very high transverse magnetization by the flux set up in the magnets 26 and then as it passes the magnets 27 the depolarizing coils set up a flux in the opposite direction. The value of the depolarizing current is adjusted by a rheostat 42 so that with no current in the coils 34 the tape will be magnetized in the opposite sense to a point on the hysteresis loop such that as the tape passes away from the magnet its flux would return to zero. Current variations in the coils 34 cause variations of the tape magnetism about such point. Alternatively, the member 69 may be made in two parts, as shown by the dotted line 94, Fig. 3, adjustable with respect to each other, so that the strong field of the magnets 26 will be partly intercepted by the field of the magnets 27 to produce a de-magnetizing effect which may be adjusted to the desired value by positioning the two parts of the member 69. The signals are recorded at constant flux for differing frequencies by correlating the impedances of the coils 34 and the output circuit of the amplifier 32, so that they are matched at high frequencies. During reproduction, the switch 31, Fig. 2, is closed downwardly so that the polarizing coils 37 and the depolarizing coils 35 are then open-circuited. The Specification as open to inspection under Sect. 91 describes also a feeding and guiding means for the record tape 11 as shown by Figs. 1 and 2 (both cancelled), Fig. 2 being a section on the line 2-2 of Fig. 1. Reels 12, 13 are rotatably mounted on a shaft 14, Fig. 2, and between the reels is a sliding collar 65 which carries clutches 15, 16, sufficient clearance being provided so that neither clutch is engaged when the collar is in its neutral position. The tape 11 is driven by frictional contact with a wheel 17 which is set at an angle to the frame 19 and loosely mounted on its shaft 29 but is normally coupled to the shaft of a drive wheel pulley by a friction clutch. The clutches 15, 16 and the clutch of the wheel 17 are all operated by a lever 54 on a shaft 61. The shaft 14 is driven from a motor 59 and rotates the shaft 29 through a belt 20 and pulley 21. During: recording or reproducing, the lever 54 is operated to engage the clutch 15 and that of the wheel 17 so that the latter pulls the tape from. the reel 13 to wind it upon the reel 12. The clutch lever 54 is afterwards moved inwardly to engage the clutch 16 and disengage the other clutches so that the tape is quickly re-wound on to the reel 13. A pointer 96, held by a spring 97 in contact with the tape on the reel 13, indicates on a scale 98 the amount of tape on. the reel 13. This subject-matter does not. appear in the Specification as accepted.