Method and apparatus 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 (10) 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 (62) 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.