Servo control apparatus for adjusting the phase of a rotary head

Abstract

A phase servo loop for the head drum of a video tape recorder employs a reference signal to initiate the production of a trapezoidal or ramp signal and a head position signal to sample and hold the values of the trapezoidal signal. The sampled and held value is employed to phase lock the head drum to the reference signal. A variable delay counter applies a variable delay to the head position signal. A flip-flop is used to detect a predetermined phase relationship between the reference signal and the head position signal. The out-put of the flip-flop controls an up-down counter to selectively increase or decrease a number stored therein according to whether or not the above predetermined phase relationship exists. The content or number in the up-down counter is preset into the variable delay counter by the head position signal which is thereafter clocked to its capacity at a high clock rate. The time for the variable delay counter to reach its capacity is thus variable in dependence on the number preset into it. A start-up circuit provides a high clock rate into the up-down counter when the equipment is first turned on to provide rapid phase lock and later reduces the clock rate to minimize jitter. When editing is being performed the number stored in the up-down counter can be held when switching from playback to recording so that no disturbance will occur in the head drum phase control. When playback is resumed, updating of the up-down counter is restarted.