In rotating polygon line scanners for non-impact printing on a moving photoreceptor, placement accuracy of the image data on the photoreceptor is a key determinant of print quality. This is achieved through the use of a PEL (picture element) clocking signal operating at a frequency equal to the number of "dots" to be written per second. By controlling both phase and frequency of this PEL clock, compensation can be made for several systemic characteristics that would otherwise produce image data distortion. Some of the principal contributors to such distortion are:
1. Variations in spinner motor speed.
2. Asynchronous relationship between scan rate and PEL clock.
3. Variation in photoreceptor speed.
4. Non-linearity of the optical scan system.
Although it is possible to design and build opto-mechanical systems in which photoreceptor feed rate and scan velocity are made linear and stable, it is more economical to use the system electronics to make the image data rate (PEL clock) accommodate the system characteristics. Photoreceptor feed rate variations alter the image aspect ratio and are typically very low frequency changes (<.05 Hz). These can be corrected by "slaving" the spinner motor speed to the photoreceptor feed rate. The spinner motor itself has speed variations (typically <2 Hz) which in turn, can be corrected by "slaving" the PEL clock to spinner motor speed. None of these variations or corrections produce any appreciable scan velocity change within a single scan. This paper will describe techniques in which electronics is used to control both the frequency and the phase of the PEL clock to correct for all of these dynamic characteristics to produce fixed spatial frequency on the photoreceptor in both scan and feed directions. Optical distortion or scan system geometry may produce variations in spot velocity within the scan line. This static distortion is identical in each sweep and is not affected by the factors discussed in this paper.
Gerald R. Paul,
"Image Data Synchronization Techniques In Rotating Polygon Scanners", Proc. SPIE 0396, Advances in Laser Scanning and Recording, (26 October 1983); doi: 10.1117/12.935271; https://doi.org/10.1117/12.935271