Ricoh VT3500, Service Manual

Page: 140 / 330

3. Original Background Correction
50 percent of the difference in voltage between the original background peak
voltage (VPH-OB) and the black peak voltage (VL = 1.7 volts) is also applied
to the current/voltage converter. In the current/voltage converter, the analog
shading distortion data are changed from an electrical current to a voltage
and added on to the 50 percent of the difference voltage.
When the original image is being scanned, the selector flips the switches
from "0" position to "1" position. The output voltage from the current/voltage
converter is applied to the high reference voltage terminal (+REF) of the A/D
converter. 1.7 volts is applied to the low reference voltage terminal (–REF).
Also, the image datum of each pixel sends to the A/D converter directly, the
image datum is then digitized to an 8-bit signal.
Since the peak hold set signal (PKHST) is low while the image is being
scanned, the lightest background datum is always held in the peak hold
circuit. This converts image analog signals to digital signals properly, even
when the light intensity of the fluorescent lamp becomes high.
NOTE: If the platen cover is dirty, lower image data is stored as a white
peak voltage due to the less reflection from the platen cover. This
means that distorted data is lower than normal data (pure white
data). As a result, white (lighter image) band will appear on prints.
VPH-OB
(100%)
VPH-OB
(100%)
50%
VL (1.7V)
50%
VL (1.7V)
1 December 1993
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