Kodak DryView 8150, Руководство по обслуживанию, Страница: 218 / 470

Service Manual
6-16
7F3318 2005 July Rev. B
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In the code sets for the other drivers, there are 32 microsteps per major step, corresponding to 6400
microsteps per revolution on a 200 step per revolution motor.
6-4-2-4. Media Lookup Table (Linear LUT)
All pixel data passes through this LUT, including the SOS level value from the Pixel Control Register that
is applied to the laser during the part of scan rotation just before the laser beam crosses over the SOS
Detector. The main function of this LUT is to convert the 12--bit image data to 14 bits. The conversion of
digital image data to the output values required by the laser occurred in the DRE Gray Scale Manager
software. The Linear LUT output values follow the video level conventions established for earlier laser
systems: a digital value of 0 produces a black spot on the film (high laser power). To produce clear areas
on the film, large digital values must be presented to the DAC by the LUT, since data inversion occurs at
the DAC. The data sense through the system is 0 = black = high laser power.
The data from the LUT is fed to a DAC that converts the 16-bit data to analog to drive the laser diode.
6-4-2-5. Laser Scanning Characteristics
Before describing the circuits on the MCS Board that transfer the image pixels to the laser assembly for
printing, it is necessary to briefly consider certain characteristics of the scanning process:
Double Scanning ----The laser scanner uses a polygon mirror with two facets. During exposure, each
revolution of the polygon causes both Facet 0 and Facet 1 to deflect the laser beam onto the film plane.
The DryView 8100 exposure engine uses double scanning. Identical pixel data is scanned onto the film
from both Facets 0 and 1. Each revolution of the polygon includes four phases:
1. The laser beam reflects off mirror Facet 0 and onto the film surface.
2. The laser beam (if allowed) reflects off the end of the mirror as it turns, scattering light. (As explained
in the next paragraph, the potential scattering of light must be cancelled out to avoid generation of
image artifacts or increased Dmin.
3. The laser beam reflects off mirror Facet 1 and onto the film surface.
4. The laser beam, if allowed, reflects off the other end of the mirror, scattering light.
Laser Dead Time ----Laser dead time is the period when the laser is turned off to avoid the scattering of
light off the mirror ends described in the preceding paragraph (phases 2 and 4). The laser is turned back
on before the next polygon facet causes the laser beam to cross the SOS detector on the Optics Module
Control Board. The length of the dead time is controlled by the Pixel Clock Generator FPGA on the MCS
Board. The LASER OFF line from the Pixel Clock Generator turns off the drive circuit from the DAC to the
laser for a period determined by a CPU-controlled register in the Pixel Clock Generator.
Pixel Stretch Correction
----The two-sided mirror rotates on a motor shaft. If the motor shaft is precisely
the same distance from both mirror surfaces, the linear velocity of beams deflected from each mirror will
be identical. Since it is impossible to achieve identical distances for the two mirror facets, the laser beams
will move at slightly different velocities and, unless corrected, the scan lines from one facet will be longer
or shorter than those from the other. The solution to this problem is to stretch one of the lines to make it
longer. The line is stretched by occasionally stretching a pixel so that the resulting line is equal in length to
the line from the other facet. This stretch parameter is measured during optics manufacturing and stored
in the Optics Control Board EEPROM.
6-4-2-6. General Pixel Flow Control Functions
The Pixel Clock Generator and FIFO FPGAs on the MCS Board work together to control the flow of image
data from the host to the laser for printing. They perform the following functions, without intervention by
the CPU: