Patent GB-964038-A (GB)
Title: Servo mechanism for video tape control
Inventor: LUTHER ARCH CLINTON
IPC: H04N5/782
Filed: 1961-04-28
Abstract: 964, 038. Automatic speed control. RADIO CORPORATION OF AMERICA. April 10, 1962 [April 28, 1961], No. 15544/61. Heading G3R [Also in Division H3] In a television recorder and reproducer, a magnetic tape 10 is driven by capstan motor 11 and is associated with a head-wheel driven by motor 15. A head 23 co-operates with a control track extending along the bottom of the tape, a 240 c.p.s. signal recovered by the head being fed through pulse-former 30 to an eight-to-one binary divider 55 providing a 30 p.p.s. pulse train to phase comparator 38. A standard sync. generator 39 provides 30 p.p.s. frame pulses from a locally generated television signal which are fed to generator 40, the trapezoid wave so produced being applied to a second input of the phase comparator 38. An error signal from the phase comparator controls the speed of the capstan motor. The binary stages 34-36 are connected to AND-gate 48 adapted to provide a pulse for resetting a bi-stable gate driver 50 the output of which is applied to gate 51 to hold the gate open to a frame pulse from circuit 19. The 30 p.p.s. frame pulses generated in circuit 19 are responsive to the video signal reproduced from the tape by head-wheel 16. Output pulses from the gate 51 are applied to the binary stages 34-36 for resetting the counter, and are also used to set the driver 50. Thus, the next pulse, in the 240 p.p.s. input pulse train from the head 23, which is received after a frame pulse from the circuit 19, causes the counter to produce an output pulse. The phase comparator 38 is responsive to the new phase of the counter output pulse to lock the frame pulses from the generator 39 in synchronism with those from pulse shaper 37. If the sample pulse produced by the counter and pulse shaper 37 occurs at a time removed from the centre of the slope of the trapezoid wave from the generator 40, an error voltage produced by the comparator 38, corresponding to the phase-difference, alters the speed of the tape until phase-coincidence is established. To correct error in the timing of vertical sync. intervals, a tone wheel 21 generates a pulse for each revolution of the head-wheel, a velocity control 102 providing an error signal when the frequency of the pulse train varies from a given value. A field pulse representing the timing of the vertical sync. interval in the reproduced television signal is applied from circuit 19 to comparator stage 104. The field pulse from generator 39 is applied through multivibrator 106 to a second input of comparator 104. The error voltage is applied to multivibrator 107 which also receives the field pulse from generator 39 whose timing is varied in accordance with the control signal from the comparator 104. A comparator 103 produces an output in accordance with any phase difference between the tone wheel pulses and those from the multivibrator 107. The respective outputs of the velocity control 102 and the comparator 103 are applied to adder 109, the composite signal being fed to sawtooth generator 114 through relay 134. The horizontal sync. pulses from generator 39 are also applied to generator 114 which produces a series of waves varying in amplitude according to the error signal from adder 109. The output of clipper 110 is applied to modulators 116, 117, acting as controllable shunts across phase shifters 119, 120, supplied from constant frequency source 118. The output signals from the phase shifters are passed through low-pass filters to a motor drive 123 generating three-phase power for the motor 15. When the vertical sync. intervals of the reproduced and locally generated television signals are substantially locked in phase, relay 134 is operated. The horizontal sync. pulses of the reproduced and locally generated signals are then compared, the error signal from phase detector 136 being fed to clipper 110.