Ricoh C211 Service Manual page 426

6-1. Shading Distortion Correction -- (Selector
2
Switch 1 ON)
Peak Hold of
Platen Cover
Waveform of
Platen Cover
.
v
PH
\
4
4-bit A/D
v
Conversion
x
?3%
?
I 1
1.7V
(Black Level)
-l
L-
When reading the platen cover to establish the shading distortion data, the high
reference voltage
(VH)
of the A/D converter is set to the peak hold level and the low
reference voltage
(VL)
is set to 535Z0of the peak hold level. Thereforej the potential dif-
ference between the output of each pixel and the 5370 level of the peak hold is converted
by the A/D converter into 4-bit data. The shading distortion data for each pixel is stored
in a memory location corresponding to that pixel.
6-2. Original Reading
Peak Hold of
VH
Background
(Corrected Waveform)
/
vp~
/
(
V, (VP" x35%z0%)
1.7V (Black Level)
1) Line Mode -- (Selector 2 Switch 3 ON)
The reference voltage (VH) of the A/D converter is set to the output of the D/A con-
verter (converted shading distortion data) for each pixel being read. Because the
reference level of the D/A converter is set to the original peak hold level, this cor-
rects for shading distortion as well as matching the output level to the original back-
ground.
Also, as described in "Black Level Correction",
VL is
set
from
O?40
to 35$Z0of the
potential difference between 1.7 volts (black level) and the peak hold of the original
background. Black level correction varies according to the original image density.
2-15A
..