High stability digital head servo for video recorders

A servo system for a capstan drive motor for use in a television recording and reproducing apparatus comprising:

• drive means for providing power to said drive motor;
• tachometer means operatively connected to a drive motor and providing train of pulses having a rate indicative of the speed of operation thereof;
• discriminator means operatively connected to said tachometer means and providing an error signal to said drive means, said error signal decreasing as the speed of said motor increases to a predetermined operating speed during start-up thereof;
• means for disconnecting said discriminating means when said drive motor has reached said predetermined operating speed;
• means for providing a reference train of pulses whose rate is nominally equal to a multiple of the vertical rate of the television signal;
• said reference producing means including a clock signal producing means for producing a train of pulses related in time to the horizontal synchronizing pulses of the television signal and having a rate that is a multiple of the horizontal rate of the television signal, and means coupled to said clock signal producing means for dividing the rate of clock pulses to the nominal rate of the reference signal;
• first comparing means operatively connected to said tachometer means for comparing the rate of said tachometer pulses and said reference pulses and providing a first error signal indicative of the time difference therebetween;
• means electrically coupling said first error signal of said comparing means to said drive means for providing a motor speed such that the reference pulses and the tachometer pulses are locked in synchronism;
• second comparing means coupled to said tachometer means for comparing the timing of said tachometer pulses with the timing of the vertical pulses in the television signal and providing a second error signal in response to time variations between the two signals, and means coupled to said dividing means for adding or subtracting clock pulses therein in accordance with said second error signal to thereby adjust the timing of said reference pulses whereby said head motor phase errors that can be caused by temperature variation, drift, loading of the drive motor and the like are corrected.