Patent GB-1007761-A (GB)
Title: Magnetic tape recording or reproducing apparatus
Inventor: BALDWIN JOHN LEWIS EDWIN
IPC: G11B15/54
Filed: 1962-02-19
Abstract: 1,007,761. Control of A.C. motors. RANKBUSH MURPHY Ltd. Feb. 18, 1963 [Feb. 19, 1962], No. 6297/62. Heading H2J. [Also in Division G3] Magnetic tape recording and reproducing apparatus comprises a synchronous motor driving a head drum, and an induction motor which drives a capstan for drawing the tape past the head drum. A periodic signal representing head-speed is derived from a disc having areas of different reflectance and mounted on the shaft of the synchronous motor, the disc being associated with a light source and photocell. The induction motor is associated with a similar optical system comprising a perforated disc. The photo-cells are each connected to units whereby the varying emissions of the cells are converted into periodic signals which are applied to a phase comparator for developing a D.C. error signal. This D.C. signal is used for dynamic braking of the induction motor or for application to an eddy-current brake which is coupled to the motor. A signal representing the head-speed may be recorded on the tape and used to indicate the tape-speed during play-back. The induction motor has two sets of three-phase stator windings energized from a single-phase source through a phase-shifting capacitor. The star-points of the two sets of windings are brought out to terminals adapted to receive the D.C. braking current. Each photo-cell may be replaced by a photo-diode 51 associated with an emitter-follower transistor 53, transistors 56, 58 forming a difference amplifier. The base of transistor 58 is biased to half the peak value of signal excursions appearing at the base of transistor 56. When transistor 58 is cut off during illumination of diode 51, a negative-going impulse appears across resistor 63 for application to terminal 65, while at other times a positive potential is applied to this terminal. A sawtooth generator circuit receives at terminal 71 the output signal from the photo-diode circuit responsive to rotation of the head drum. Transistors 72, 75 form a multi-vibrator triggered by a positivegoing signal to provide a positive-going pulse across resistor 83. When transistor 85 receives a positive-going pulse, it is cut off for a period whereby transistor 89 discharges capacitor 93 to initiate a stroke of the sawtooth waveform. When transistor 89 is cut off, capacitor 93 is charged through transistor 94 for applying a potential to the base of transistor 98 connected in series with transistors 99. The sawtooth potential at the junction of transistors 98, 99 is fed to output terminal 107 for application to the comparator circuit. Feedback through resistor 108 stabilizes the amplitude of the sawtooth voltage. The comparator circuit receives at terminal 111 the signal developed by the capstandrive photo-cell during recording or from the control track during playback. The negativegoing components of this signal cause transistor 112 to pass current and initiate the operation of a blocking oscillator comprising transistor 113 feeding drive pulses for a four-diode switch 122. The sawtooth voltage is applied to the bridge 122 from terminal 123, the potential across capacitor 124 varying with the relative phase between the drive pulses and the sawtooth voltage. The error signal is applied through D.C. amplifier 125 to the means for producing the retarding torque on the capstan drive motor. Specification 972,132 is referred to.