Ricoh PRIPORT VT 2000 Series Service Manual page 54

2.5.6 A/D Conversion
OS
VT2100/VT2130/VT2150 Circuit:
Amp
VPH of Platten/
Original
Background
VPH of Shading
Distortion
Memory
Black Level
Correction
Output of
Selector 1
Shading Distortion Correction (Photo Mode)
Shading Distortion Correction (Line Mode)
Photo Mode
Line Mode
This circuit converts the analog input signal (V
using both the white level reference (V
In each mode, the settings of the "X" and "Y" switches in Selector 2 are
automatically determined by the PHOTO and SHMRT signals as shown in
the above table.
VH
(White Standard)
4 bit A/D
65%
Convertion
VT
(Black Standard)
35%
VPH x 80%
VPH x 53%
VPH x 6%
PHOTO
SHMRT
Back Ground (White Peak)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
A B
Selector 2
SW
0
1
X
2
3
0
1
Y
2
3
A B
PHOTO SHMRT
L
H
L
H
) and the black level reference (V
H
2-27
A
1
BAD1
BAD1
BAD2 1
BAD2
0
BAD3
BAD3
1
BAD4
BAD4
A/D CONVERSION
Selector 2 (X,
L SW0 ON
L SW1 ON
H SW2 ON
H SW3 ON
) to a 4-bit digital signal
S
28 February '91
B
0
0
0
0
VS
VH
M1
M2
VL
Y)
).
L