Patent GB-676761-A (GB)
Title: Magnetic recording and reproducing
IPC: G11B5/70
Filed: 1947-06-02
Abstract: 676,761. Magnetic sound records and recording. BRUSH DEVELOPMENT CO. June 2, 1947. [July 20, 1946] No. 14547/47. Class 40 (ii). A magnetic recorder employs a recording medium coated or impregnated with permanently magnetisable particles, a transducer head having pole portions separated by a non-magnetic gap not greater than 15 microns, and a bias supply of frequency greater than the highest to be recorded and magnitude such that output voltage on playback is a maximum. In Fig. 1 a magnetic recording tape 31 is fed during recording and playback from a reel 32 to a reel 34 past an erasing head 35 and a transducing head 36 mounted on a common support 37. Suitable mechanism (not shown) provided for winding and rewinding is associated with a limit switch 40 and a pulley 38 combined with a flywheel to maintain constant tape speed. With a switch 42 in the recording position as shown the erasing head 35 is energized from a H.F. source 44 so that the tape reaching the transducing head is magnetically neutral. The output of a microphone 45, after amplification by a recording pre-amplifier 46 and a main amplifier 47 and combination with an H.F. bias of approximately three times the highest recording frequency, obtained from a potentiometer 48 across source 44, energizes transducing head 36 for recording. With switch 42 in the opposite, playback, position, source 44 is switched off and the output of transducing head 36, after amplification by a reproducing pre-amplifier 51 and the main amplifier 47 energizes a loudspeaker 52. The recording and reproducing pre-amplifiers are designed to give a uniform overall response at all frequencies. If this is not essential one or both pre-amplifiers may be omitted. Alternatively, entirely separate amplifiers may be used for recording and playback. Transducer and erasing heads. These may be formed as in Fig. 2A by two strips of highly permeable sheet metal 2-21, on opposite sides of a rigid member 2-27, having a non magnetic working gap 2-23 adjacent the recording tape 31 and a similar gap 2-23-A for symmetry. To produce the gaps, which are of the order of 15 microns, the poles may be in direct contact or separated by very thin shims. The windings are in two sections 2-22, the symmetrical arrangement reducing the effect of stray fields. The poles may be strengthened with stiffer material and methods of producing the heads are described. To ensure positive and uniform contact between the tape and the heads with less tape tension, resilient pads 57, Fig. 1, e.g. of felt, urged by springs 56 may press the tape against the heads. Such pads may be arranged to be lifted off the tape during high speed rewind. In an alternative arrangement, Fig. 2A, a resilient layer 57 e.g. rubber or felt is supported by a rigid member 56 and the transducing head is mounted on a supporting arm (not shown) spring biassed towards the tape to flex it as shown. To avoid non uniformities at the edges of the tape and to avoid wearing grooves in the heads, the tape is wider than the poles and is guided, Fig. 7 (not shown), to a central position. To ensure efficient erasing under these conditions the erasing head pole width is greater than the transducing head pole width. Recording medium. This comprises finely powdered permanently magnetisable particles, e.g. magnetite, - ferric oxide or cobalt oxide powder, having a particle size of the order of 1 micron, held dispersed in a layer of bonding compound, e.g. a synthetic resin, approximately 25 microns thick. The medium is deposited in liquid solvent form on the surface of a backing sheet or base of relatively strong material, e.g. paper or plastic film, the evaporation of the solvent leaving the layer united to the base. The specification includes much data relating to the production of suitable recording media. There are references to : (a) Heat treatment of thermoplastic bonding compounds to reduce abrasion of the heads by the surface of the recording medium. (b) Addition of alkaline materials to the magnetic material to avoid reaction with bonding compounds containing chlorine. (c) Splicing the recording medium by cement or heat. (d) Solvents which do not easily dissolve the base material.. (e) Recording layers on both sides of a tape. (f) Recording layer on the margin of a motion picture film. (g) A motion picture film backed with a coating of light reflecting material, e.g. aluminium paint, from one side of which the picture is reflected and on the other side of which the recording layer is deposited. (h) Non-magnetic coloured labels indicating the start or finish of a recording. (i) A conducting layer associated with a relay to stop or start recording or playback. (j) A pencil or marking tool attached to the recording head to apply a distinguishing mark at the start or finish of a recording. (k) Coated threads of high tensile strength, e.g. synthetic resin filament or fibre, used as the base which may be first coated or impregnated with a thin layer of resinous compound before applying the bonded magnetic powder layer.