One of the major requirements of a TV tape recorder is precise timing. Even small timing errors
cause picture distortion, flutter and loss of synchronization in the reproduced picture. To minimize these
errors, closed-loop servo systems are used to control the motors that drive the drumwheel and the
capstan. In a closed-loop servo system, automatic control is achieved by feedback from the output of
the system. The feedback is compared with either the input or a standard reference in an error detector
and the resulting error signal is used to control the output in a manner that tends to minimize the error.
In the recorder the drum assembly servo system controls the speed and phasing of the drumwheel
so that the video information is properly recorded and the capstan servo system controls the longitudinal
tape speed via the capstan; hence, the video and associated longitudinal tracks are recorded at a uniform
rate. To accomplish this, the drumwheel servo locks the speed and phase of the drumwheel motor by
comparing a pulse derived from a pulse generator coil on the headwheel shaft with a reference pulse
occurring at the television field rate.