Method for placing information on a medium while compensating for deviations from a characteristic time period

摘要

A laser scanning apparatus is provided with a compensating arrangement whereby deviations of a synchronous drive are compensated for by adjusting the frequency with which the laser beam is modulated. The laser beam is directed onto a rotatable spinner which causes the laser beam to scan a synchronized moving medium in a number of sequential and parallel lines, each of which is determined during a cycle of the movement of the spinner. The spinner is driven by a synchronous motor. During each cycle, a period error is measured to determine a compensating signal which is held for the next successive cycle and is used to control the frequency of a variable oscillator which determines the clocking rate at which information is employed to modulate the laser beam. According to the method of the invention, lines of information are placed on a synchronized moving medium responsive to the aforesaid laser beam during successive cycles. The method involves modulating the laser beam in accordance with the information and at a frequency which is modified for each cycle in accordance with the deviation of the preceding cycle with respect to a characteristic normal time period.