Patent GB-676766-A (GB)
Title: Magnetic recording and reproducing
IPC: G11B5/02
Filed: 1948-02-13
Abstract: 676,766. Magnetic sound recording and reproducing apparatus. BRUSH DEVELOPMENT CO. Feb. 13, 1948. [Dec. 5, 1946] No. 4259/48. Class 40 (ii). In a magnetic recording and reproducing system, the recording head is energized by a constant frequency alternating current during recording and gradually de-energized at the end of a recording period. In one embodiment, a supply reel 21 feeds the record track 31 past an erasing head 32 and recording/reproducing head 33,. capstan 23 and limit switch 25 to a take-up reel 22. Felt pads 40 urge the record track 31 into contact with the heads 32, 33 and separate driving motors 41, 42, 43 are associated with the supply reel 21, take-up reel 22 and capstan 23 respectively. The switch assembly 35 controls the various functions of the apparatus being provided with pushbuttons 50 ... 55. The inward action of one push-button causes all the others to be released to the outward position. In the playback outward position of push-button 55, reproducing head 33 is connected to the input of amplifier 70 through switch-blade 61, the amplifier feeding the loud-speaker 74 through auxiliary amplifier 72. In the record inward position of push-button 55, the microphone 76 is connected to amplifier 70 through switch-blade 62 and the amplifier output applied to the recording head 33 through auxiliary amplifier 78 and switchblade 61. A high-frequency oscillator 80 has its output applied to the erase head 32 power being supplied thereto from rectifier 82 through switch-blade 63. High-frequency bias for the recording head 32 is derived from potential divider 88 across the output of oscillator 80. Switch-blade 63 also energizes the auxiliary amplifier 78, and may also de-energize the amplifier 72 if it is not desired to monitor the recording. When the capstan motor 43 is energized by operation of push-button 51, it also energizes relay '92 if button 55 is in the record position and switch-blade 64 is closed. The contacts 94 of relay 92 are thereby opened to prevent the full output of oscillator 80 being applied to recording head 33 through contacts 95 which during recording are closed by a cam 97 on push-rod 55. When motor 43 is de-energized by withdrawal of button 51, relay 92 is de-energized and the full output of oscillator 80 is applied to head 33 to demagnetize the record track 31 in the region following the record. If recording is stopped by withdrawal of push-button 55, relay 92 is deenergized at contacts 64 but contacts 95 open only after a time delay introduced by dashpot 98 so that the oscillator output is applied to the recording head 33 for a short time subsequent to the recording. Alternatively, the cam 97 may be so shaped as to open contacts 95 after contacts 64 have opened. A condenser 96 connected across the oscillator H.T. supply causes the oscillator 80 to provide an output for a short time after the H.T. supply is broken at contact 62. The decaying oscillations produce effective demagnetization of the record track after the record section. In another embodiment of the invention, a record/playback switch is biased towards the playback position, being latched mechanically when moved to the record position. Termination of a recording operation energizes a latchrelease magnet and the switch returns to the playback position. In doing so it passes through a position which applies the full output of the erasing and biasing oscillator to the recording head but with a decaying characteristic introduced by a time-constant circuit in the grid circuit of an output amplifier valve which is returning to its cut-off condition; (Fig. 2 not shown). In a third embodiment, in which a common sound transducer is used as a microphone and loud-speaker, the source of demagnetizing current is permanently connected to the recording-reproducing head. When the record/playback switch is returned to the playback position the H.T. supply to the demagnetizing oscillator is interrupted but a charged condenser ensures a continuing decaying output therefrom, (Fig. 3 not shown). Specifications 647,962 and 676,761 are referred to. The Specification, as open to inspection under Sect. 91 comprises also an embodiment in which decaying oscillations are provided by connecting a capacitance across the winding of the recorder head to form a resonant :circuit tuned to the oscillator frequency, which generates a damped train of oscillations when energized by the demagnetizing surge. In another arrangement for providing the decaying train of oscillations an unbalanced bridge balanced up as two of its arms heat. up with the passage of oscillatory current, and a further circuit comprises a condenser which is charged from the H.T. supply and discharged through the recording head. This subject-matter does not appear in the Specification as accepted.