| 摘要 |
<p>949,212. Automatic speed control. ELECTRIC & MUSICAL INDUSTRIES Ltd. May 19, 1960 [May 19, 1959], No. 16844/59. Heading G3R. A magnetic tape 1 is driven past a transducing head 2 capable of both recording signals on an auxiliary track and deriving signals therefrom, a rotatable mount 3 carrying a plurality of heads for recording or reproducing video signals, each of said heads co-operating with each of successive groups of tracks on the tape transverse to the direction of movement thereof. When the apparatus is set for recording video signals, synchronous motor 4 for driving the mount is operated at 400 c.p.s. and rotates at 200 revolutions per second. An optical or magnetic transducing head 5, detecting the speed of the motor, derives an oscillation having a frequency of 200 c.p.s. and this is applied through amplifier 7 to the head 2 for recording the signal on the auxiliary track. When the apparatus is set for reproduction, the head 2 receives the recorded signal whilst the head 5 derives an oscillation, corresponding to the speed and phase of motor 4, which is amplified to lock a sawtooth waveform of half the frequency in generator 9. The output signal from amplifier 8 is compared with regard to phase with the oscillation from head 5 in a phase detector 6 the output of which comprises a D.C. error signal applied to phase shift device 10. The output of this device consists of a train of pulses corresponding to the points where the D.C. error signal intersects the longer descending portions of the sawtooth waveform from the generator 9. The pulses are applied to a circuit which produces a sine wave and is tuned to 400 c.p.s., and phase modulates the waveform from this circuit. This waveform is applied through amplifier 13 to the motor 4 which is locked into phase with the recorded reference signal. Since the sawtooth has a periodicity which is one half of that of the mount, the maximum mechanical phase adjustment obtainable is in excess of 180 degrees, the mechanical phase shift being double the electrical phase shift. The circuit 10 comprises valves V1, V2, the sawtooth from generator 9 being applied to terminal 15. The D.C. error signal is applied with opposite polarity to terminals 14 and thence to the control electrode of valve V1. The signal at the anode of valve V1 is of pulse form and is applied through capacitor C1 to the control grid of valve V2 affording a sinusoidal output tuned to 400 c.p.s. and having the correct driving frequency for the motor 4. Specification 919, 481 is referred to.</p> |
