| 摘要 |
<p>An arithmetic frequency synthesizer having an accumulator for recursively accumulating a R-bit long input word at a predetermined reference frequency to generate an adjustable-frequency bit clock for a flying spot scanner with a rotary prism(12) for cyclically scanning an intensity-modulated light beam(15) across a photosensitive record medium(16) includes means for aritmetically modulating the value of the accumulator input word on a scan cycle-by-scan cycle basis for adjusting the bit clock frequency to compensate for prism motor hunt errors. More particularly, for modulating the accumulator input word value in accordance with such errors, there is a counter(41) which is incremented or decremented during each scan cycle at a frequency which is scaled to the reference frequency, whereby the count accumulated by the counter during each scan cycle is a measure of the average angular speed of the prism during that cycle. This count, or the average of the counts accumulated during a plurality of scan cycles, is offset by a calibrated R-bit long bias value; either by preloading the bias value, in whole or part, into the counter at the start of each scan cycle, or by summing the bias value, in whole or part, with the accumulated count. The bias value, in turn, is selected so that offset count, or the averaged offset count accumulated during one or more earlier scan cycles, respectively, defines the modulated input word value which, when applied to the accumulator at the start of the next scan cycle, adjusts the frequency of the bit clock to compensate substantially for the average motor hunt occurring during the earlier scan cycle or cycles.</p> |
