Ricoh VT3500, Service Manual

Page: 138 / 330

[Reference Data Correction]
1. Shading Distortion Memory
The platen cover is uniform in color and
in reflection. However, the waveform of
the white platen cover from the CCD
shown on the right, is due to distortions
of the light path.
During the first five line scans, the selector flips the switches to the "0"
position. The standard white peak voltage (VPH-SW) in the peak hold circuit
is applied to the high reference voltage terminal (+REF) of the A/D converter.
50 percent of the difference in voltage between the peak voltage and the
black peak level voltage (VL, 1.7 volts) is applied to the low reference voltage
terminal (–REF). Also during this period, the platen cover white data are
directly sent to the A/D converter. Then, the upper 50 percent of the platen
white cover data is digitalized into 8-bit signal form. This corrects the data
which is distorted by more than 50 percent of peak voltage. They are stored
in memory as the shading distortion data.
SHMRT
VPH
0
1
0
1
PKHTM
PKHST
50%
1.7V
8 bit
8 bit
8bit
+REF REF
_
VR(+) VR( )
_
Image Data
A/D
Converter
Selector
Peak Hold
Circuit
Current/
Voltage
Converter
Shading
Distortion
Memory
D/A
Converter
To Image
Processing
PCB
VPH-SW
100%
50%
8-bit
VL (1.7V)
Memory
1 line
1 December 1993
2-102